I use it- I also downloaded their toolbar so it is easier to make drops on other people's entrecards. Basically it will load 10 sites at a time (in what every category you want) so you can make the drops quicker.
I am kind of experimenting with it right now and seeing if it actually will bring me any long-term traffic. I am not sure- but I hope to continue making 50-300 drops a day for the next couple months. ( I do it on my laptop while I am watching tv)
Basically how it words is that you put their widget in you page and it will allow other people to "drop a card" on you- essentially driving traffic to you site- but not necessarily traffic that actually reads your blog.
I love Entre card. I also downloaded the toolbar to make dropping on sites easier. I think of it as dropping my blog business card on someones blog/website. It makes me look at websites that I probably would have never gone to or found. I get 10-15 drops on my Entre Card inbox right now but I am new to it. I warn you though it can be very addicting! Its like a goal for me to drop as many as my 2 boys let me!
I wrote a blog post about Entre Card back in October. Here is the link:
Entrecard is basically, a traffic exchange. You will get traffic based upon the amount of traffic you give. No hard and fast rule there, but most people notice you get 1 or 2 visits for every 3 blogs you visit initially. Depending on how you use it, your ratio will increase or decrease. If you drop on a lot of blogs consistently, a lot of bloggers will drop on your blog in turn. If your dropping decreases, the amount of drops on your blog will also decrease. That just seems to be what happens.
Meanwhile, the drops (points) you earn allow you to purchase advertising spots on other EntreCard blogs.
Like already suggested, read the eBook. It's very informative.