I think it is alright. I have it on my site but Im thinking about taking it off. Its not converting anything for me. They just come by to drop there card so they get something out of it.
Google suggests that you don't use any type of traffic exchange plans, this includes Entrecards, etc... Google prefers natural link building, search engine traffic and direct type in. In any case, if you're looking to increase your traffic because of adsense and you're considering Entrecards, let me say that this is a very bad idea. Why? Basically that is the type of traffic that just won't convert, Entrecard visitors don't click ads, and even if they do they don't really convert (I mean take an action within the advertisers site) What will this mean? Well, basically you will be smartpriced because you have poor quality traffic and will loose even more $$$