I have done around 5k on an R1100GS, and a few hundred on both the 1150GS and 1200GS. Oh and I did over 150k on an 1100RS, 1100RT and a number of 1150RT and 1200RT
I never quite got on with the 1100/1150GS, despite loving my old R80GS and R100GS Paris-Dakar, as well as highly appreciating the other 1100/1150/1200 boxers: The oil-cooled GS always felt awkward and unsettlingly balanced. The 1200GS is already a lot more "together", I still much prefer the road-going Beemers.
The TDM 900 is again a whole different beast altogether. The boxer is a lot "choppier" (some would say characterful - I would, for example) and pushes you forward from lower speeds like a big invisible hand. The TDM isn't actually too bad either at lower revs but gets more lively where the Beemer (esp the 1100/1150) starts to run out of breath: It's nearly scary how well the Tedium goes when you revving it well over 6000 rpm. It's such a pity the out-dated injection messes up to lower-midrange so much, the (in)famous airbox fix doesn't seem to fix that for everyone (it actually made things worse on my '08 model, so I reverted it back to standard). Sitting position and suspension makes the TDM unnecessarily unexciting, it's got the power and surprisingly modest weight to be a lot more fun than it is - just as much as a Beemer R of F series.
You can take a TDM off-road, but to about the same effect as doing so with a Suzuki SV-S, a Honda Hornet or a Vauxhall Astra. Don't get fooled by people who park a TDM in a field to take picture of it, of post videos taking it over a muddy road at 15 mph: It will never fly, and if you try, you will pay dearly for it.The GS, provided you get a proper training, like the fantastic BMW Enduro Park in Hechlingen or Wales, with its better weight distribution, more tractible low-down power delivery and rotating mass, longer suspension and more appropriate wheel-sizes is a much better bet. Nothing like a single-cylinder off-roader though, unless you are a Stephan Peterhansel.
Typical problem with boxers, again from a rather lenghty experience: leaky crankshaft oil-seal, oiling up the clutch. Cheap item but a lot of work to fix (wheel, drive and gearbox need to come off). Shaft drive is great but higher unsprung weight mean less sensitve suspension. The engines last long but both drive and gear-box are mechanically brittle, from private experience as well as professional and as bloodrunner volunteer. Needless to say that repairs on drive-train can be hugely expensive. For that reason, I would personnally never buy a used BMW boxer without warranty. Having said that, it seems that the more recent 1200 are finally living up to BMWs reliability record again: I hope the malaise with mechanical problems and poor build-quality that started with the then all-new R100RS in '93 isn't starting all over again with the all-new water-cooled engine planned for 2012/2013.