(August, 2007: The review has now been substantially revised to be relevant to version 2 of Pianoteq.)
First off, all those looking for the Pianoteq to jump out of the box emulating their favorite piano will be disappointed. The company clearly states that instead of trying to nail a particular make of piano, they tried to integrate a bit of the best of everything into the sound. I would say they more or less succeeded (and it probably was a smart move, too, considering the amount of flak anyone claiming to perfectly replicate a Steinway Model B in code would take!) As of version 2, there now are two "modern classical" piano models -- C1 and C2 -- with C2 being the one in active development. Each 2.x version has brought something new to Pianoteq's sonic arsenal, primarily on display through the C2 presets.
You can alter almost each and every aspect of the sound to your liking. You can change everything from the tuning to the size of the piano to the stiffness of its hammers and soundboard. In one small 10MB application, you have access to an almost infinite variety of piano-like instruments. Try that with modern day samplesets, and you wouldn't be close even after packing a couple of harddrives full of samples.
As with all physical modelling, giving the user control over every little parameter would be both overwhelming and probably not musically useful: In addition to the parameters available for tweaking through the user interface, there are also underlying, hidden parameters that define the basic characteristics of the piano (whatever they are) that can't be tweaked by the user. These are defined in "models" -- some of which are built into the main program, and some which are freely downloadable from the Modartt website. So when designing your own presets, you should take some time to choose the most suitable model to base your new sound on.
There is a vast difference between the sound of version 2 and the original. Whereas version 1 only sounded acoustically credible if you got your head around thinking of your monitor speakers and surroundings as part of the piano casing(!), version 2 sports an all-new soundboard model that brings the wooden box into the sound. This gives the sound a true "recorded in a room" air, making it more familiar to those used to ambience-recorded sample sets. I'd even go so far as to say they might have overdone it a bit: Depending on you personal preferences, you may just think it sounds a bit woolly now. Still, heaps better then the "ears right up against the strings in an anechoic chamber" feel of the original, and nothing a bit of proper, room-adapted EQing doesn't fix in most cases. (A multi-node graphic EQ is included in the price of admission, by the way, as is a functional, but not top notch, reverb.)
PLAYABILITY AND "SPECIAL EFFECTS"
This is where all the competition can pretty much pack up and go home. The last crop of sample players, like K2 with its scripting and convolution abilities, are only just beginning to nibble at the edges of the features already tightly integrated in the Pianoteq: The way each repeated note changes in timbre depending on its state when it is re-struck, the way you can pedal-catch notes, and half-pedal etc. (given a progressive sustain pedal and -capable controller). Not to mention that there is no such thing as velocity-dependent samples involved - everything is smoothly gradual, all the way from note velocity 1 to 127! Still no sign of slackening lead here: While you still struggle to find even a half-decent progressive sustain implementation in the competition, these guys are tweaking away at things like half-pedalling sympathetic resonance. Kudos.
The actual mechanical noises representing the piano action may not be to everyones liking, however - but all can be switched off or at least very much attenuated.
While version 1 was only available as a plug-in, version 2 can run as a stand-alone application, which is very convenient for this type of solo instrument. An extra bonus is the 1-track MIDI recording/playback sequencer. (When you get tired of doing all the work yourself, just load one of the thousands of piano-roll MIDI files available on the net, sit back and enjoy!)
To sum up, let's put it this way: I may well use sampled piano sounds in final studio productions in the future (although the need is radically less than it was with version 1) but I can hardly see me going back to samples when practising piano playing, or even when recording piano parts. And that's from someone who is perhaps an experienced, but not even a good piano player. Only good enough to feel the difference between a living, breathing instrument and something just trying to be.