A good few years ago I was asking myself the very same question- I had a sportsbike - albeit an old one (FZR1000 Exup, but it had been played with

) and wanted something a bit different. I originally thought about a R1100S Boxercup but couldn't quite stretch to it at the time, and spotted a 5yr old mint 1150GS with less than 8k miles on it at Williams BMW. I never test rode it, but liked the look of it and took the plunge.
Coming off a sportsbike with close on 130bhp to an aircooled heavy lump that wobbled it idle and would run out of puff really easy was a bit of a culture shock, and my first impression of it was f*ck- what have I done??
The following weekend, myself and Mrs Haydw went to the lakes. It was a revelation- I could ride the roads without been thrown around, and actually used torque instead of revs. After a year or so, we had our first foreign holiday on it. I was totally converted.
My 1150 now has 58k on it from holidays, work, days out- its survived being nearly drowned, and have had the unpleasant experience of a Ford Ka trying to write it off with me still aboard. About 16 months ago I bought a 1200GSA, but Mrs Haydw insisted that we keep the 1150. The 1200GSA is different to the 1150 in many ways, and I cannot choose between them. The 1200 has given me no end of grief and financial headaches lately, but I have just been very unlucky. Been out the last two days for a bimble on it and I have fallen in love with it all over again.
If you are still of the sportsbike mind, then don't discount an 1150- any saving you make against a 1200 can be used for a sportsbike. You will find though that for most of the time you will end up with the GS and wonder why you have a sportsbike sat doing nowt in the garage.