I've been using Entrecard for a few days now and the traffic sent from them is pretty decent, checking over my google stats it's number 5 on my traffic sources.
The traffic has mainly come from I guess people searching for sites to advertise on, I haven't bought many advertising spots myself, taking things slow. My feeling is that traffic would be next to useless as they are probably looking mostly for if your site is worth their credits.
The whole deal reminds of sites like blogexplosion so far, I guess once I have enough credits to buy ad spots on some bigger sites I'll get a good picture on if it's worth the time.
My advice for Entrecard is to move slowly once you've signed up, all my ad spots were full within minutes of signing up, 5 in total all worth 2 credits each for a days advertising, I've been rejecting them ever since and now a days advertising on my blog will cost you 22 credits a day.
Take it slow, because it looks like the sharks on it already know the ins and outs of it, kind of taking advantage of the newer users, though not in a mean way.