You're right about the velocity layers, there are none in Pianoteq (other than the midi standard's 127 possible values). A sampled piano simply uses pre-recorded notes at different velocities, and plays back the corresponding recording (sample) for each note you play.
Pianoteq only uses samples for things like keyboard mechanic noises and the such, where modeling them would make absolutely no difference. It calculates all other aspects of the sound in real-time mathematically form a huge set of parameters, many of which you can tweak with the interface. It took years for this approach to become viable, but in my personal opinion, version 4 is when it started to be actually viable for a believable piano sound. It offers a lot more control over the sound than samples, you can control elements that a sampled piano would need entire different sets of samples.
For example, if you want to have a realistic sound of the same piano with lid open or closed, you'd need samples of the piano with the lid open, and an entire other set with the lid closed (or fake the effect with some lowpass/eq) - at minimum. With pianoteq, there are tons of parameters, like the length of the piano, the hardness of the soundboard, hardness of the hammers, etc etc. that you can change on the fly, and the notes that are playing at that exact moment will continue resonating with the new sound.
Another thing, very important in the "connected" feel you mentioned, is that pianoteq doesn't render single notes, it renders the piano sound as a whole. This lets it much more realistically do things like for example sympathetic resonance (strings resonating from other notes), duplex scale (the part of the strings outside the "nominal length") and so on.
The Modartt team has worked very hard on this product, and they stand behind it. In my experience their customer support has been beyond excellent so far.
http://soundcloud.com/delt01Pianoteq 5 STD+blüthner, Renoise 3 • Roland FP-4F + M-Audio Keystation 88es
Intel i5@3.4GHz, 16GB • Linux Mint xfce 64bit
