If you are just after pianos, there are a couple of NI interfaced plug ins that are piano based and I think they may be a bit better.... but you'd have to listen to the demos to decide.
I have GPO... and a piano, so I don't use it for the piano. The instrument samples are good but as Garry mentioned, may take a bit of time to learn. For example, you must have some controller data (mod wheel) on the track to get volume.... On another website, a number of people mentioned some clicks and pops in the samples..... the GPO people stated they were unable to reproduce the problem and the people did report they were on different samples (one guy had problems with the oboe anothe with the trombone). It is not a loop point issue and may have to do with utilizing the resources of Sonar. I have not had those issues and am pleased with the purchase.
Being a bit of a software.... okay JUNKIE (there are you happy now), I also have EWQLSilver. It is about the same price and has a little different spin. For example, in GPO you build your woodwind and brass ensambles with individual "players") in EWQLSilver, the ensembles are already built. I like them both and would try to listen to demos of both. I am not sure why, but GPO seems to be a bit more popular. Gary Garritan is very responsive with help and feedback, so that is a definite plus.
IMHO, GPO is a good purchase.
Take Care