I had better chime in here, as 4 weeks ago I traded my Aprilia Futura for a new 1150GS Adventure...... I owned my Ash black example for 18 months, did 10k on it many trips in the UK and a euro trip to Italy. It performed flawlessly. I traded because I wanted something with better dealer backup, and better for long trips as I got bit by the touring bug in a big way. I didn't want a huge tourer so thouht I would try out one of these monster traillies. I made the right decision.....
First off, have a test drive of the 1150 as well, because it isn't just the old model GS, its a completely different bike. I didn't like the 1200 at all when I rode it because it just didn't have the presence on the road of the 1150 and didn't quite feel as sure footed which is probalby due to the weight diff. All this is IMHO, go and have a go of both, back to back 1/2 day each if poss.
I'll give you some positives and some negs
Positives
* Way more comfortable, upright riding position is excellent
* Wide bars = fun?!!! More leverage means you can sling it into corners, you wouldn't beileve were possible on such an ungainly bike.
* Road Holding. I kid you not. With me riding, the GS out handles my Futura on 99% of the roads I normally ride. I expect it to lose out on the big fast A roads, but I haven't had a good sample yet.
* Vision. Being up high is awesome! It sounds obvious, but because you can see over cars, you can go round em so easy.
* Wind Protection. The protection on GS is superior to the Futura in terms of your head. Yours boots are more exposed though.
* After market! There is one! fettling your Fut is like pulling teeth as the quantity of aftermarket kit is limited
* Dealer Backup: BMW isn't likely to run into financial difficulty and leave you high and dry for spare parts.... like sprockets!
Negatives (I guess some of these are Adventure rather than the new 1200)
* Gearbox. Oh dear, its clunky and not as nice as the Aprilia. It is however direct and you get used to it.
* Power. No powerband take off at 6k any more
* Weight. OMG its big! When on the move its fine, but manuevering can be tricky.
*Cleaning. A huge amount of nooks and crannies + spoked wheels! - it takes forever to clean.
That will do for now
I am well chuffed with this new bike, and itching in a big way to get on the continent

Whatever you decide I'm sure you will be happy.
Before I forget, do you know of
www.apriliaforum.com ? There is a UK Futura owners club of sorts which I'm part of (still am) and we are having a Futura + lots of other kit to up Helmesly and around the moors up to Whitby. Will be a great day out. Its happening on the 11th of this month. Great group of guys. Have a looksie.