What are the stats for # of impressions they get versus signature campaign?
That is hard to say for sure. If you were to try a signature campaign on a trial run you could probably insert a goo.gl link to measure how many times people click on your ads by participants verses how many people click on your forum ads.
You will generally pay on a per post basis for signature advertisements, so if your campaign is big enough you could have more then one advertisement displayed on the same thread. On the flip side however if someone visits a particular thread more then once they probably are not going to notice/look at the forum ad at all and your ad could be displayed when this happens (it would be possible that in this scenario, someone wearing your signature happened to post the last post, so in this case they would see your signature ad but not your forum ad).
It looks like that bit-x spent roughly 10.28
BTC this month (January) for roughly 10,200 posts.
Here is their signature campaign thread, and
here is their google doc spreadsheet detailing how much each participant has posted (look at the tabs on the bottom).
Here is a thread detailing all the current open signature advertisement deals currently open.
