While I do try to avoid them, I've been caught in some pretty nasty downpours over the last 3 years; though somehow I almost completely missed them last year on the 650R, I rode through varying degrees of rain on the 250R back in 2010.
As long as you keep your wits about you, and remember not to do any sudden/jerky movements or go crazy on brake/throttle actions, you should be fine. I rode quite regularly on the highways to & from Port Hope area and such, and never had an issue in the rain. Watch out for stuff like train tracks or other metal surfaces, especially while turning, but otherwise ride as you usually would (perhaps a bit more conservatively) and you'll be alright.
That said, it can be scary, mostly because of other traffic on the road or other hazards. Had an 18-wheeler whose back-end was fishtailing scare the crap out of me on the 401 coming around a bend. I was in the passing lane trying to get by him, but the 250R lacked the acceleration I needed to GTFO when it happened -- I wound up having to ride along the left edge of the lane to avoid being sideswiped by his trailer.
