For what its worth, I went from a V-twin 650 to a Yamaha XT250 and love it. I have never had an issue with power going up hills or on the road. However I weigh 150 pounds. What I like the most is the fact that the bike is so light that when I dump it in the bush alone, its not too tough to pick it up. I have dumped it in some pretty rough stops and the fact that the bike was only 270 pounds was a hugh help - especially since I was in the middle of no-where and alone.
I really like the size of the bike. I am a very happy camper with mine and doubt that I will ever move to a large bike like a BMW or KLZ 650.