Once you actually get it running, you'll iterate through a hundred deployments before working out all the quirks and bugs. In the end, I actually got it working but, somewhat ironically, didn't end up using it because managing it was more difficult than just deploying and scaling manually. Kind of sad if you ask me.
Your experience might be different if you choose to pay Scalr for their services, since they know all the quirks and probably have fantastic internal documentation. They, understandably, don't have much motivation to help you use the software yourself.
They were supposed to be the "cost-effective" alternative to RightScale, but I'd pay $500/month to get out of using Scalr. All of my money is riding on Amazon's own load-balancing/auto-scaling solution that should be coming out in the next 6 months to one year.
Sorry if this post is overly negative...I got my YC rejection letter this morning.