I am actually trying out elves and FoL right now. It's not really that great at all.
Hidden Paths is an expensive tech, you need to beeline for it and you've got nothing but warriors before then, for about 120-ish turns on normal speed. Founding fellowship of leaves is a gambit that could mean many turns of lack of development, even if you get it, and you'd be rather hosed if you don't. You're not counting on treants right?
Hidden Paths is not even that useful, as useful GP farm and floodplains areas don't have many forests. You have fewer, larger cities, so I guess it's good for builder games. But then you can't really expand outwards either, because you don't have siege engines, and waiting for mages with fireballs would mean yet more technologies. Mages aren't even the best defensive units around, although they do have other useful spells. Oh wait - bloom is not one of them. To grow forests in your lands would require incense and priests of leaves, which is rare and probably doesn't exist in your territory, and might even be removed once blight hits. Plus it requires priesthood, yet more expensive techs. Not to mention that they still don't provide any military benefits.
It's also high upkeep, which means you won't be able to support a large empire with it anyway. Of course, once you do have a large empire, you would have more hapiness and health resources around, so you probably could abondon it for foreign trade anyway. Or rather, you could abondon FoL and go for another religion instead.
The benefits of the religion just doesn't seem to pull together.