http://www.advrider.com/forums/showthread.php?t=329337
You can read about the Yamaha WR250R on above thread on ADV Rider.
Few talk about it here except TMotten. 40K posts there, lots of info. I've read most of it ... but lost interest in the bike ....
Did a short 20 minute test ride, ON and OFF road. Impressive.
Not sure the WR is the "perfect" RTW travel bike for everyone, but it's damn GOOD ... both ON and OFF road. But, I've read of surprising reliability issues in above ADV thread. Unusual for Yamaha. But mostly all good reports.
One would have to pack really light on ANY 250. I owned a WR250F (race bike) until recently, owned several 250's in the past. My Fav was former XR250R. Amazing bike, didn't travel RTW on it but did some long-ish rides.
No doubt, once off road in bad conditions a 250 would be a holiday compared to even a well set up 650. Much easier riding.
But realistically, how much true off road nasties will you go looking for? Most travelers take a "road" somewhere, not a trail. A well set up 650 can handle most roads. Mud and Sand? Advantage 250.
The WR-R is a tiny bike, fit me perfect (at 5'7"). The owner I borrowed it from is 6'2", he looked funny on the thing ... "Like A Monkey Humping A Football"
And what will happen to it's "light, nimble handling" once loaded up for RTW riding? My guess is it will go to Hell unless very carefully modified. (suspension? stabilizer? braced sub frame?)
250 class bikes (WR-R may be the BEST out there) have advantages beyond riding off road:
Positives:
1. Great fuel economy/Range (bigger tank needed on WR-R)
2. Great in congested cities, filtering, curb jumping, riding down sidewalk, easy parking and getting into Hostels/Hotels.
3. Relatively easy maintenance, long intervals. 250's have LONG service life on Chains, Sprockets, Tires. (This is BIG for a traveler IMO

)
4. Easy and FUN to ride off road. Easy(er) to pick up.
Negatives:
1. May feel "strained" at speeds over 60 mph, weak in mountains. Lack of power/torque riding over 2000 meters. (In S. America, riders often go to 4K meters, stay there ALL DAY) All bikes lose power up high, a 250 is badly affected by this.
2. Limited load carrying ability, possible broken sub frame or frame if overloaded or if luggage packed improperly and ridden HARD off road.
3. Very small bike, not Large Rider friendly, less leg room, narrow seat, suspension would need upgrading for heavy rider with gear.
4. Not 100% bullet proof over long distance (WR250R). Quite a few negative reports on BIG ADV Rider WR-R thread. But about 80% of posts there are cheer leaders for the WR-R. (as expected)
5. F.I.: Blessing and Curse. Blessing: GREAT fuel economy, performance and ability to compensate at altitude, Curse because F.I. is very battery dependent and if even ONE component fails or is corroded ... you are DONE. (NOTE: this is very unlikely as most modern elec. components will never fail)
Curse because F.I. also depends on reliability of a high pressure fuel pump. Bad fuel/dirty/watered fuel can sometimes DESTROY the fuel pump. (filtering fuel would solve most of these woes)
As suggested, ride a few bikes, see which ones speak to you. Be sure to imagine that bike with 30 kgs. of luggage/extras strapped onto it.

250's Rock! Baja, XR250R