Magpie is an ad network for Twitter. Advertisers sign up and create campaigns which consist of magpie-tweets (i.e. ads). Twitterers allow Magpie to post magpie tweets among their tweets and for them to get paid for it.
I have tested the Magpie service and it does what it says and gives you back information on click-thrus from the people who have posted your ad.
I think it could do more to give an idea what is going on with your campaign while it is running.
The click-thru rate was pretty good but I haven't gone back to fund another campaign as I think I can get the same results myself.
It's worth a try if you're interested in twitter advertising but I'm pretty sure I won't be using it again.