Depending on how well you play it, it's like finding somewhere between $20 and $100, free, to spend on advertising your web comic.
If you like it, you can stay and make EntreCard part of your promotional arsenal.
If you don't like it, or don't get results, you can walk away, having given it a fair shot.
To put it in perspective, an average user would be happy earning 20 - 100 credits in a day. That's enough to buy a bunch of lower-priced ads. Because you are a web comics creator, for a limited time, you can get 3000 just for joining.
I recommend you read my previous two posts so that you have the best shot at a happy experience, but there's nothing stopping you from joining, blitzing the web with ads, and then deciding what to do next, if anything.
The equivalent offer on TV or radio would be something like 10 free credits, with a catch or two. I wouldn't keep talking about this if it wasn't very appealing and good for web comics.
Now, go join, so I can return to reviewing and commentary. Thanks. (When you do, send me your comic's URL so I can send you instructions to get your coupon for 3000 points, which is simple. My email is at right.)