How is it working with Cubase and Logic? I went to the Tascam web site and found pages of updates and improvements bringing us to version 3.10. Powercore and UAD plug ins are suppose to work now which was needed. I was thinking about getting Halion, but with the upgrade cost for Giga 3 from 2.5, you get so much more sampled orchestra sounds compared to the cost of Halion alone.

Do I still have to uninstall 2.5 for 3.10 to work?

you must uninstall 2.5 ,and yeap its worth it if you're into big libraries .
Giga 3 can play bigger projects,more polyphony and it sounds better somehow .Ofcourse dedicated machine for giga is best solution ,but it can work with seqencer on one machine if you're carefull and you're not overloading it .If you're into synth/sampler editing ,filters/effects then Kontakt2 or smth is still better solution.If you're into huge string,piano,drum libraries with not much editing of sounds Giga is damn powerfull .

enjoy