ThaumCraft?

It's different from other mods - very creative and could be fun to play... just not a good fit with stuff like BC, IC and RP. I have, however, used it with Equivalent Exchange and they go together nicely.
In short - a great mod, but not the style I want to play here.

Thaumcraft is a fun mod with IC2, BC2, and EqEx, It is easily abusable to get resources for IC2 and BC2 using EqEx.
Thaumcraft + EqEx = endless resource.....err well EqEx is already limitless resources but adding Thaumcraft to it is overkill. *hint hint* duplicators

Well as silly as this sounds just because you have the ability doesn't mean you have to abuse it.
I use IC2, BC, Forestry, Minecolony, EE, Thaumcraft, Twilight Forest, Railcraft, Steves Carts, Powercraft, various and sundry addons for those and some other mods (and only 1 block id to spare or I would have more).
I don't build 15 square chunk megafactorys. On the other hand, I've got so much stuff I have barely scratched the surface of most of it. Some of the mods are almost redundant but I still havent decided which I like best (IC2 and Minecolony are must-haves for me, Twilight Forest adds another dimension even if I haven't gotten around to exploring it, Powercraft redstone logic is a must since I don't use Redpower) I can't decide if I love or hate BC, Forestry rubber tree plantations alone make it a keeper but there is a lot good to say about it.
EE I haven't gotten around to learning the new version but it seems it has been rebalanced a lot and it is cool and unique.

So anyway, Thaumcraft. I like it. A lot. The ability to have multiple portal networks alone makes it a keeper. The other stuff I haven't had a chance to get into much but they sound good on paper, the mod is pretty unique and well done (although I have to say that about most of the Forge mods in general), my major annoyance (getting pwned by wisps constantly) has been fixed, and the next version is in the works so it is still being added to and improved.

So yeah, it is definitly worth downloading.