Other then the previously mentioned Biomass output increase, the logs can be squeezed to give Liquid Force, and the leaves occasionally drop Force Nuggets. A good start for any player using DartCraft and Forestry is to get that squeezer going, as if you're using 1.6.X you'll need as much liquid force as you can get. Otherwise it's just nice to have.
As an aside, anyone else think it's cool that one can use the Carpenter to make force ingots? It takes less "force" which is nice. Not that it's a big deal when you have a decent Force Tree Farm, but still, it's helpful.
Also, if you're on 1.6.X you'll want to smelt some force logs and make some Force Sticks with Force Planks to make some super awesome Force Torches. They are slightly brighter(15 vs. 14) and can be upgraded.(put Bane on those suckers and put 'em on your roof. No more Spiders on the roof! Might want to add a cleanup system though...) In addition Force Torches can be shot from a Force Bow, which is pretty damned handy. If that also applies to Upgraded Torches, I'd be in Heaven, but I haven't tested that, nor do I believe in the traditional Christian Heaven. Or any of that Jazz, but that's off topic.