Intelligent, forest-guarding trees? That's ent-irely original.
A treant's plant RHD means it has all kinds of spiffy immunities. Sadly, they also mean it has plant RHD, which is a lot like an aristocrat's but with worse skills. Anything making up for these seven levels of suck?
The answer: kind of. Treants are Huge, with +18 to strength and a not-insignificant bonus to constitution as well. All mental stats are boosted somewhat, too. -2 dexterity is annoying, but not very. The fire vulnerability, however, is a major problem that's going to need compensating for in some way.
Treants also have +13 natural armor (quite good), DR 10/slashing (nice), Trample (meh), and the ability to deal double damage against objects with full attacks (which seems kind of unnecessary in most cases).
Their main ability is Animate Trees, which effectively creates two slower treants under the treant's control. Ask your DM whether they stick around after the control wears off (RAW implies they do) and whether they have the ability to animate trees themselves. I recommend finding some way to move them with you, because the full round it takes before they join a fight and the restricted range of biomes the ability functions in mean the trees won't be that useful otherwise.
Another important question to ask your DM is going to be whether treants can use items. They do seem to have fingers in the picture, and there's undoubtedly nothing quite as awesome as a 30-foot-tree swinging an enormous sword around, but it could be argued they can't quite manipulate objects.
In the end, I think that treants deserve +0 LA. They have some major disadvantages, but the createable tree-servants are pretty neat and the high strength, huge size, and ability to wield weapons (you know, probably) make up for a lot of them.