I used to use ValueClick, but I was incredibly disappointed by their low quality of advertisers. (Nearly as bad as FastClick's) In addition, all ads were run-of-network (untargeted) and mostly CPC, so there was very little revenue generated.
Recovering from the holiday season and a bit strapped for cash, I decided to give them another shot with my unsold inventory. And I am happily impressed :) On the first of 2005 they rolled out an entirely new Publisher UI which is pretty darn impressive. In addition, they junked all of their junk advertisers and now are mostly showing ads for fortune 500 companies.
If you tried ValueClick in the past and were disappointed, I suggest you give them another shot :)
The only big problem with them is at this time they only allow NOSCRIPT defaults. Therefore, if you use other networks, they have to be last tier, as you can't give their defaults to another ad agency's script. A big disappointment for a media network that owns companies such as CJ, BeFree, etc. among others. They are the only ad network I know of that doesn't offer this. :(