This is the subject that high volume players are considering these days. There have been countless numbers of posts on this subject at 2+2 and poker blogs (including this great one by suckout).
My Party account was originally signed up with no affilate (how little I knew about the online poker business back then......), and I had been only using it strictly for bonus whoring. All of my other play at Party was through a skin where I was receiving rakeback.
When the skins were broken off, I looked for a few offers from affliates that would quietly give rakeback. The offers I received were tiered between 20-25% rakeback depending on MGR.
I finally got around to emailing Party's VIP group to get an offer directly from them. Their initial offer was $200 for 7000k raked hands, and I "negotiated" to $350 for 10k (about a 22.5%/raked hand improvement). If I play above this threshold, my bonus will be increased on a pro-rata basis. This seems to be a fairly standard offer given the stakes and # hands I play when compared to what others were listing in this 2+2 thread.
I asked and was told that the offer is only for November, and that some different, but better (yeah right...), promotion scheme will be coming for their VIP club.
The question I ponder is if I should try to get a combination of rakeback plus bonus $. I think for now I will take the conservative approach to stick with my existing Party account for just the bonus $ and see how others fare.