don't bother, all it really does is make your textures slightly lower quality (not really much fps boost)

Please learn to use the search feature and post in a relevant thread: http://teamfortress.tv/forum/thread/12811/1

If you're using global settings, you need to "save" them after modifying in the lower left corner of the window. For profiles, though, saving settings is not needed, apparently. Also, settings only apply when you launch TF2; so, you need to configure whatever you're overriding or injecting, save it (if you're not on a profile), and then launch tf2 once you're done.

I use it for the SMAA injector, which is delicious. It looks better than MSAA, which doesn't seem to work right all the time in TF2, failing on certain things like some floor-wall edges.

worked for me when I followed the instructions
and stimpack, it's not just about fps gain. Some people prefer the look and simple aesthetic of tf2 with that enabled. Although the nvidia equivalent is so much more damn sexy