Not sure why anyone would pay for it. I requested an invite directly from Google (for free) on Tuesday, got it on Thursday. Frankly, I don't find it so impressive but that's just my opinion.
I got one too but I haven't even bothered to go through their process of registering, I think they require you to first use the code on your phone with their app and then one can use it on desktop. Don't know what's the fuss is about but isn't it just another mailbox? Or is there something I don't know..