Please note that I'm writing this as someone who went looking for a piano-first instrument. My review is based off of this requirement. Also, the Yamaha I'm comparing it to is a mid-range ($1,200) composer/workstation keyboard with semi-weighted keys, so please take this into account.
I have been working with the Casio PX-3 for about a week now, and it's going back. I had hoped that it would be a relatively low-cost alternative to a Kawai, Yamaha or Roland DP, but it just isn't. To preface my review, I must say that my current home situation mandates that I use headphones fairly often, and this is, perhaps, where the problem lies (then again, many of the other commenters here indicate they use headphones often with this DP, so, it might just be my ears). Also, I play a lot of slower ballads that require large chord voicings, particularly in the midrange, so if you play a lot of ragtime, rock, etc., you might not hear any of this.
In general, I like to feel of the pseudo-ivory keys under my fingers. The key action is a bit mushy and heavy and they keys are a bit noisy and "thunk-y," but I don't mind this aspect of the piano as it falls within my normal boundary of acceptability. The controls are easy enough to work, all things considered, but they are a bit small and lack good tactile feedback. The decay on the piano sounds is a bit abrupt, but changing some of the custom settings helped this somewhat (but not totally to my satisfaction). Also, the note falloff under sustain pedal seemed far, far too abrupt, as well, but there might be a way to change that.
The piano sounds themselves are fair/good (nothing to write home about--again, under slower playing conditions), but some of the piano notes sound electronic-y to me, particularly in the midrange and during a sustain. While my Yamaha's piano sounds ring and resonate as they start to fade away, the Casio's sound like they're buzzing (it's somewhat video-gamey in nature; I guess this doesn't matter a ton except on the last chord/voicing of a phrase or the entire song?). Things also start to get a but muffled during complex chord voicing, even in the midrange (where I normally want chords to be well defined). My Yamaha seems to shine under the strain of complex chords while the PX-3 seems to falter. All of these things I could probably live with for $800, though.
What killed this DP for me was the amount of distortion behind the notes, particularly in the first B, C, and C# keys above middle C. If one turns the brilliance to 3, it's really, really noticeable. It's almost like the sampling rate used to record was much lower for these keys that all the rest. Put another way, while most of the other keys sound like MP3s sampled around 192, those three sound like they were sampled at around 96-112. The pronounced hissing behind these notes is terribly bad, even without the harsh distortion.
I noticed this initially through my speakers--I thought these two notes were simply hitting the harmonic resonance frequency of my speakerboxes at first (my speakers are quite good, so why I thought they had any time of resonance in this sense is beyond me). Unfortunately, clearly this wasn't the case after I put on some cheap headphones. Things got downright nasty after I put on my Ultrasone closed-back studio headphones. I played all of the other keys one-by-one to see if this very high level of distortion was isolated and it generally was; however, there's noticeable distortion behind all of the notes on the PX-3 which makes the overall reproduction of the piano sound a bit harsh (my Yamaha workstation also has this tendency, but it's far, far less pronounced--strangely, the worst distortion on my Yamaha comes from these same very three keys; what's up with that?? Even these three keys on my Yamaha at their worst is still better than the Casio's other keys at their best). I might be able to live with the harshness if I played faster songs or on a stage where detail isn't as important. More to the points, though, the distorted B, C, and C# keys are so incredibly bad that one can easily hear it even during complex chord voicings. If it were on one of the higher or lower sets of B, C, and C# keys, it would be alright, but since these are the set above middle C, I use them quite a bit. I even tried to convince myself that I could work around these three keys...but, yeah, that's when I realized I was trying to rationalize this far too much.
Now that I've heard this distortion, it can't be unheard. I noticed this same level of distortion in roughly 3/4 of the piano sounds. I didn't bother with the rest since I purchased this as a piano-first instrument. The distortion is exacerbated by turning the brilliance up to 3. The distortion can be mitigated by turning the brilliance down (especially to -3) and changing the attack to about 5 or 6. However, this leaves the piano sounds very muted and muffled, so this isn't really a good work-around for me.
If you're playing on stage or in a church, my guess is there will be so many other sound artifacts running around, you'd never notice this distortion. In these instances you might be playing through these notes quickly, anyhow, which will cut back on the distortion that seems to happen in these circumstances. If, on the other hand, you've purchased it to be used at home and you like to play songs with a slower pace or if you hold chords in your right hand for at least a beat, just be aware that this might be a problem.
I'm trying to be as even-handed as possible here--I'm the kind of person that got rid of a one-year old car because of a constant ticking noise it developed. No one, not even my SO, claimed they could hear it. It was there, though, and I wasn't crazy, and sounds like that simply bother me to death. Be sure to take this into account when reading this review. Also, you simply might not be bothered by the distortion at all. For $800 compared to $1,000 or more, most people could probably live with a few quirks. I wish I was one of those people sometimes.. Frankly, if you've heard this, I'd appreciate it if you'd leave a comment to let me know what your take is on this.
Given all of that, if you're looking for a light, portable, and budget stage piano, you should certainly give this a shot. As always, see if you can play it in the store before bringing it home. With this model, it would also be a good idea to bring some closed-back headphones with you, as well.