I recently moved from a GT JTDM to the V6 GT and I've got to say it was probably one of the best choices I have ever made.
It depends on personal circumstance but for me I don't get to do nearly as much driving as I used to now, so having the diesel isn't as important. I'm getting somewhere around the 30-32mpg mark at the moment on the motorway with the V6 so consumption really depends on how heavy your right foot is, although it's always very difficult to resist the urge to open it up, the sound is incredible. Certainly not subtle though - I pulled up into my office carpark last week and everyone in my office (on the third floor with the windows closed) heard me coming.
I have the connect nav+ system with new map discs, it's a very good system although you can't search by post code, which sort of out dates it, even though the maps are totally up to date. The onboard phone is also pretty good although no bluetooth. All I've done is get a free pay as you go sim from O2 and now my car has it's own telephone number. Whenever I get in the car I switch my mobile to automatically redirect (about three button clicks) and all the calls come in through the car phone and you can answer them with a touch of the steering wheel controls.
The V6s came with a Bose sound system as standard which is excellent (it's also standard on a lot of the rest of the GT range), but I tend to keep the music turned down and have the windows open so that I can hear the engine, much better than my (or my girlfriend's..) music collection!
Just be aware that things like belt change services are very expensive so if you can, make sure you find a car that has already had it done (and recently!). It's also worth bearing in mind that there is a batch of V6 GTs in the UK with the wrong emissions figures on the paperwork, which is well worth spending a little longer hunting for given that it means cheaper road tax!
Overall, a lot of car for not a huge amount of money and given that they weren't offered for that long and only in relatively small numbers, they are already well sought-after and will only become more so.