My wifes mukluk with 3.8" wide tires (a few posts up) climbs like an f'ing mountain goat. Granted I can definitely feel it on some of the longer climbs, with the added traction I hammer up some steep climbs faster than i could dream about on my other bike which is a nicely built sub 25lb 29er 1x9.
*I also only ride 10-15 miles at a time. Any longer than that and I can see the added friction/inertia starting to take it's toll. I've heard of guys racing these bikes solely for the purpose of staying away from Class I races and still being at the top of Class II.... slows them down juuuuust enough, and fun!