Oh yeah baby, you wont regret getting a GSA. If the budget can stretch to a twin cam model then go for that one, post September 2010 don't have the silly fuel strip. The ones with premium and dynamic packages are better if you are going to use it as touring machine otherwise get a bog standard one for going off road on long trips or don't fancy the lectricatry. I've got a 2009 model and I have added a few bits that makes touring on it better for me and the missus (intercom, Hella FF50 fog and spot lights, mudsling, grip puppies to name a few). I didn't mess around with the screen as it is perfect for me at 5'10''. The seat has come in for criticism but for me I just put the front on the high setting and rear on low. The riding position perks you up high so the view over the traffic makes you a bit quicker. 33 liters of fuel goes a long way, but you'll only notice the difference in weight just after you have filled up. The tyres on sale for the GS are all good, it is just down to personal preference. You will scrape the foot pegs and the centre stand, and maybe the pannier and sometimes the rocker cover protector. It is fantastic on a track day

. Mine has toppled over a few times but the damage is superficial and cosmetic. Rocker covers for a pair is about £140. The original BMW ones is good as I found out exiting Agostini's at Snetterton, lost the rear under power and slid on the rocker cover. I don't bother with a top box neither with the Touratwat add-ons, the roll bag is more versatile when touring and the bits I added is for practical reasons. It is good for 50% of us as it left the factory. There are a lot of horror stories regarding the final drive, EWS, fuel strip (like mine) but most owners are very satisfied with the bike. The reliability issues of the earlier models has been sorted, sort of as there is still the odd problem here and there. Don't let that put you of considering that BMW sold hundreds of thousands of the 1200GS.
Mine has done 18000mls and the issues I had were:
4 x fuel strips ( 2 under warranty, 2 BMW good will)
Clutch slave cylinder leaked (BMW good will, I paid for the part which was cheap considering it is a full days labour)
Apart from regular servicing it has been a good bike and I enjoy it. The R1100S hasn't been used that often but I decided to ride it more this year.
What else do I want to do to it? Get it remapped by Hilltop Motorcycles.
What can be done to it? Every thing you can think of, 21'' or 17'' front wheel conversion, 180 or 190 section rear tyre (R1200S bits), GS120 kit from Nippy Normans, Off road, touring, instructing, track days, commuting, you name it you can do it with a 1200GS.