I don't know how I came across this site, but after stumbling upon it I'm completely addicted and haven't stopped playing it. It's very easy to play, just jump right in.
It's a lot of fun, but I'm really embarrassed at how bad I am. I'd probably have a panic attack if I tried to play against someone else, so I'm just doing the single player right now.
My only (minor) complaint is that you can't really register at the site with an email address, so if you want to change your password or something like that there's no way to do it.