I have Gigastudio 2.5/160 connected to my Yamaha CVP900. It produces vastly superior piano sounds, and there is a huge library available of just about every high-end piano you can imagine, as well as other instruments. For realism it's a whole different world.
Here are some things to know:
- It takes a fast computer with at least 756MB of memory, two fast hard disks, a good sound card and a midi interface. Unless you are handy with computers I'd advise getting help in configuring something. I would not go with an out-of-the-box Dell or similar consumer machine
- By the time you have the computer, the software and a few libraries you're into 2-3 thousand dollars
- It's best not to run any other software on the computer - streaming from a hard disk takes all of the cycles you can get. If you try to run, say, a sequencer at the same time, you'll risk clicks and pops.
- I've never had a midi latency problem playing live, even with 160 voices
- There are other soft samplers - Kontakt/Kompakt from Native Instruments for example. Each has passionate advocates and detractors. I think the consensus is that Gigastudio has the best streaming for large samples but others are superior in other ways - user interface, effects. etc. This summer should see a new release of GigaStudio 3.0 which sounds like it will set the new standard
Summary - better sounds, more complicated, expensive. A very good source of information and help with this technology is at NorthernSounds: http://www.northernsounds.com/forum/index.php?
Many of the sample developers hang out there and have web pages where you can hear their libraries.
Hope that helps, let me know if you have specific questions.