Update (for those like me that play solo piano):

I was quite serious about getting into sample libraries two years ago, and in the process, found this interesting site:

http://www.purgatorycreek.com/

The tests of each sample and/or digital piano was created with a short midi file (which can be downloaded from the site).

At the time, I was really impressed with the sound from some of the better ones; Synthogy Ivory and EastWest Quantum Leap Pianos.
Tonight I visited the Purgatory Creek site again, and downloaded some mp3 files and compared them with the file generated by Pianoteq from the midi file noted above (pianotest.mid). These are the ones I compared (I used my wave editor to load all seven files at once for easy comparison):
Ivory Bosendorfer
Yamaha Digital Piano Gran Touch
Bardstown Bosendorfer
EastWest QL Steinway
EastWest Bosendorfer
Native Instruments Akoustik Piano - Bechstein
pianotest1 with Pianoteq

Compared to the Pianoteq rendering, some were quite bad, and some like the Akoustik were actually wretched sounding. The vaunted Ivory Bosendorfer doesn't stack up to Pianoteq.

Earlier today, I went back to some of the library sites and downloaded samples of some classical piano so I could compare them with the same pieces I generated with midi files and Pianoteq. The selections I used were the Debussey's Arabesque and Claire de Lune, and Beethoven's Pathetique (2nd movement) and the Moonlight.

I was literally stunned at the difference between the sampled renditions and the Pianoteq ones. The selections created with the libray samples were simply not of the quality of the ones I created with Pianoteq.