Pianoteq is a different kind of piano sampler because it's not actually a piano sampler at all. Instead it's a piano "modeler." I don't know exactly what that means technically but what I do know is that it sounds more realistic than samplers do to my ears and it also takes up a lot less space on the computer!
On top of the very realistic sound that you can get with this VST instrument, there's also the great variety of ways that you can adjust the sound (you can change the tuning, the mic position, you can choose from a large variety of different pianos.)
But what you really want to hear is an example of what it sounds like. That's why I'm including Beethoven's "Moonlight Sonata" as played by Pianoteq below. I've actually only got Pianoteq 2.3 (Pianoteq 3 is the latest version and it's what's available now) and I still think it sounds amazing. In fact, I think it sounds so good that I haven't bothered upgrading to Pianoteq 3 even though it's supposed to be even better.
***EDIT***
Due to a bogus copyright claim by WMG, the PianoTeq version of "Moonlight Sonata" was taken down by YouTube. Apparently it sounded so realistic that they thought it was actually a recording of a pianist playing "Moonlight Sonata" that they have under copyright. I think that's actually pretty good proof of how great PianoTeq sounds.
Obviously the song itself (being a Beethoven composition that is about 200 years old) is not under copyright, so the only explanation is that they thought the recording itself was under their copyright. Of course this wasn't actually a recording at all. It was the PianoTeq software "playing" the song.
***END EDIT***
I recommend the SL-990 weighted MIDI keyboard to take full advantage of the power of this piano modeler software. A huge part of what makes Pianoteq sound so real is that there's basically an unlimited amount of variety in the velocity (which is different from samplers which have a limited variety as far as velocity goes) and of course you need a weighted keyboard to take advantage of that aspect. The SL-990 is what I use and I love it.