Todd and Christina,
I have just purchased one here in Oz for a bit of Adventure riding, popular here because of the following:
Light and manouverable
Relatively fuel efficient
easy to repair
Heaps of accessories available. Oil coolers, tanks etc etc available.
Best set up with soft luggage I would think. That is what I will be using...
Did heaps (6 months) of research before deciding. Looked at KLR/KLX/XT/XR. The engine has been around since the ark in one form or another and yes Lois did both Africa and South America on versions. A bit more road orientated than say a XR250R and if you are not to fussy about top end speed.
Trans Africa I guess can be done on anything with two wheels and I think has been. Keep up the oil changes and you shouldn't have an issue. Sump only holds 1100mls. (extra with oil cooler available from Yamaha)
I like them because I am a bit of a light weight both in size and they way I tend to travel with gear.
Attached website should give you some idea as to reliability etc:
YAMAHA TTR250 TRAIL and OFFROAD MOTORCYCLE FAQ
Heaps of info out there on the net and on various other sites.