I just kind of poke around on a few of their boards, with two exceptions. I really like reading the Eberron forum. Detailed information from the creator and lots of really good campaign ideas.
That being said, I consider the Character Optimization (CO) board to be a holy grail. As a DM I get to find out about potentially game-breaking combinations before they happen. I also get ideas for really, really unique and different NPCs (the only catch being that I have to tone them down a bit). And no one, not anyone, knows the rules better than CO regulars. As a player, the ideas for characters are endless. You just have to take the board with a grain of salt, since the posters are admirably intense about it.
If you ever feel like taking the D&D Rules forum here to the extreme, I highly recommend checking it out.
-Talgian