I use it on my forum and it is alright. If you want to make big money out of it then you need a very big membercase or guests. I placed my ads in the header. I personally prefer buysellads over adsense and use adsense as a backfill when your ads are not sold. I thought this was more effective than just adsense on its own.
I've had it up on a couple of my sites in the past, didn't really make any money out of it though (too small user base for it to actually give some revenue).
Also since I don't really like seeing a lot ads on other sites I did try to keep them as unintrusive as possible and to a minimum which I suppose could have contributed to the low income.
I know you can earn from them though under the right circumstances so I might go back to setting a few ads up on some places.
I've only ever used it on blogs as an actual source of income - I always find forums don't tend to get much income via PPC ads like Adsense so there is no real point in running them.
I've used adsense. I think it is a realy good way to make some profit out of your site. Mostly there will be ads showing that are relevant for your members so the chance they click on the ad is bigger > more profit for you.
For the few times I have had it on a forum, I tended to get enough to satisfy server expenses. Though, I never really tried to get the forum to the point, where Adsense was worth being on there all the time.
I think it's effective but you'll need to have a steady flow of traffic/users before really seeing any substantial earnings, so people should work on that first.