Annother nod for the GT-6. As has been stated, some of the stuff is fairly useless, at least at my experience level, but there are a ton of good features. Lots of good distortions, amazing cleans and a great, great acoustic sim. There are about 6 different distortion models that sound really good out of the box, and if you really want to delve into tweaking them, you can get into some pretty deep stuff. You can also build custom amp combo's combining several different pre-amps. The effects are nice as well, chorus, delay, flange...a ton of stuff.
It is also great for practicing thru headphones...the sound quality is amazing.
Annother bonus is that when I record and mike my JCM 800, I run the mike through the GT-6 (set on tuner bypass) and then out to my computer. I can set the amp as loud as I want, and then control the volume out to the computer via the volume pedal on the GT-6.