In my testing I found anything over 35# on the bike was just too much to enjoy an off road trip.
At 35# my bike felt unstable on decents and was a real bitch on climbs.
Picts from one of my 35# test runs. Last Fall I hauled my winter pulk kit up with the mtn bike for a shake down overnite.
So for me about 25# is all I want to carry on my bike.
http://www.flickr.com/photos/outandback2012/sets/72157631963376234/I have a Nashbar knockoff Bob trailer but only use it in the Desert (around Lake Powell) flat dirt road riding where you carry 5 gal of water.
I would not want to use a trailer on hilly singletrack.
hth