Great Value, Good Digital Piano
The Yamaha YPG-635 is an excellent value. There are a few things I think Yamaha could have done better, and thus the four star rating. In summary: Good sound, great keyboard, great value.

The Good:

This keyboard has excellent and realistic keyboard action. I played on several at a music store to see which of the various brands felt best. To my fingers at least, none of the keyboards in the price range felt any better (and even many that cost more didn't feel as real). It's easily within the normal variation of real piano keyboards. This was the key deciding factor for picking this keyboard.

The Grand Piano voice is good, easily acceptable. There are better sounding piano voices; typically found only in instruments at twice the price.

The general MIDI spread of voices is decent, and many are top quality. The extended voices are very good as well, but are obviously synthetic. Two of the deciding factors for me were the 64 note polyphony and the general MIDI compatibility. I play a lot of MIDI files and my old keyboard was not general MIDI compatible (it had the voices on the wrong patches). Many songs overrun the 28 note polyphony of my old synth; they do not on the YPG. I'm sure I could create sequences that need more that 64 note polyphony, but for the most part, 64 is enough (128 would be great, though).

For a complex keyboard, most of the functions are easy to use. For the complex things, there is a menu selection system and scroll wheel that I've found works fairly well. When I'm actually playing the keyboard and not running MIDI files through it, I prefer to use the "Grand Piano" button and have it simply be a piano. Easy.

The Value is amazing. There are cheaper keyboards with as many voices. There are keyboards with as nice or nicer sounding voices. The combination of price and voices, at least for me, meets in the middle on this instrument.

The USB-to-HOST port and associated drivers work with Windows 7 (64 bit). I was worried as my previous MIDI interface doesn't. I have several MIDI capable devices and was planning to replace my no-longer-compatible MIDI interface box (and still may since I still have the other MIDI devices). See my note in the "the Bad" section about the lack of standard MIDI ports.

The Neither Good nor Bad:

OK, the title for this section is a little humorous (I know, very little). The points I want to make, though, fit in neither the good nor bad, but are comments on advertising hype and expectations.

The advertising claims the Piano voice is the best... their words are: "You're never more than a button push away from the most realistic piano sound in portable keyboards today." I have to quibble with two parts. This keyboard, at 70 pounds or so, isn't all that portable (unless you're a professional musician with Roadies to move your stuff), and the sound is good, but not the best since Yamaha's pro-Line portable stage piano's have even better sound (along with much higher price tags).

The display isn't big. They advertise playing songs from the score display. You probably won't. It's big enough you could and small enough you won't want to.

The wood doesn't seem to be real wood, but more like the particle board with veneer like that of cheap furniture (well, it's a fairly cheap digital piano, so I guess that fits, huh?). However, it is solid (and 70 pounds or so).

The auto-accompaniment isn't as feature rich as I expected moving up from my ancient PSR synth. There are more choices, but fewer controls over them. Perhaps I just haven't learned them yet.

The Bad:

As mentioned a few times in other reviews, the volume isn't impressive. A real piano would easily drown out the YPG audio output. Not an issue if you listen on headphones or are in a small room - just be aware, a large room, like a church or club, will swallow the sound of the keyboard and not even burp. Of course, if you want, you could add amplified speakers or feed it to a PA, but there will be a signal to noise issue since you're using a headphone jack as the keyboard does not have Line-Out jacks. I realize the YPG isn't a pro-line instrument and Line-out is a pro-level feature.

Speaking of the headphone jack, it's on the back side of the keyboard, behind the music rest. A really awkward spot if you ask me. On the front would be much better. This is a minor deficiency.

While we're on the back of the instrument, the USB to Device (for flash drives) is on the back of the keyboard and hard to get to if you have the keyboard near a wall. I bought a 30 inch USB Extension cable so I can plug and unplug my flash drive more easily. Not a show stopper, and not an issue if you don't use a flash drive, but how hard would it have been to put it on the front.

The MIDI implementation is Yamaha's USB to Host connection. You can't use a MIDI controller to use the sounds of the YPG, or use the YPG to control an external tone module unless you have a computer handy - it doesn't have the standard 5 pin DIN MIDI ports. I thought long and hard about this before deciding I could live without standard MIDI ports. If you need the standard MIDI jacks, think about another keyboard.

Conclusion:

The YPG-635 is an excellent value, you do get what you pay for and there are a couple of compromises and quirks at this price level. The combinations of features, though, are well worth the price. The sound is good, the keyboard action very good, and the good points far out-weigh the bad points.

It doesn't come with a cover, but Amazon sells one that fits: Musician's Gear 88 Key Stretchy Keyboard Cover.

Perfect for a nice family piano
I ended up buying this after looking around for a month and recommending it to my Father. I was happy enough with his, and found a really good deal on it, and ended up buying one myself.

This is just a really nice piano. It has a really good feel (probably the best touch feel until you jump up another level in price), the piano sounds are very warm and realistic, the rest of the sounds are all very good (some of the strings/horns aren't quite there, but come on, who really cares, you bought a piano, not a saxaphone). The included pedal does an adequate job, though I'm probably going to pick up with optional 3-pedal unit at some point. The speakers are very loud and produce a good sound. There is a headphone jack, as well as a USB port for MIDI-control.

Overall, this is a great digital piano. It looks really nice, and is well constructed. The sounds are very good for this price-point, as is the feel. Does the sound/feel get better? Absolutely; just be prepared to pay 2-3 times as much. For anyone that just wants a nice piano for the family to have, one that won't go out of tune, feels good, is full size, and has a nice sound, you can't find much better than this! I would definitely recommend it to others!

Good piano
This is a very nice piano for the price. The quality of the sound is not identical to the real piano, but it's better than any other electronic instrument I've used so far. I'm also happy with the quality of materials used, and it's very easy to assemble. The selection of other instruments is diverse and you can get very creative with it. I bought it for personal pleasure rather than for professional use, and it works just fine for this purpose. One thing I didn't realize is that it's still not a full-size piano, one octave is missing compared to the real thing. I thought 88 notes was it, but I was wrong. Other than that, I'm happy with the purchase.

Best thousand bucks I've ever spent.
Feels and sounds like a really good piano, especially with a quality set of headphones. Perfect for practicing on at all times of the day without driving your room mates / neighbors / family up the wall. A much cheaper and more portable alternative to having a legit piano. My biggest complaint is that after playing on my Yamaha for a year, I am now spoiled, and a lot of the real pianos that I get to play on from time to time feel / sound like absolute garbage.

Great Piano for the price
My brother did a lot of research on these pianos and decided on this one. I took his word for it, and have not been dissapointed. Simple to use, and I love the quick button to return all setting to the basic Grand Piano setting. I am not a big musician, so I don't have much experience with all that it has to offer, but it appears to be very comprehensive. The keys are weighted the same as my regular piano. I wanted to get rid of the big bulky piano... And it has not been missed at all. And you can't beat the price for this quality.

The only CON would be that I can't find a cover for it. My wife is going to make one for it.