After being a customer for the Rackspace Cloud (formerly Mosso) for quite some time, I'm happy to say that my business and anyone who listens to our advice will never be using this hosting service, ever again.
Rackspace is an amazing company. They are know for having great servers, great support, great everything. You can't beat them. Mosso was a side project that was swallowed up by them which aims to run websites in a real, actual cloud. This is a valiant cause. To be able to upload a site to one server and have it scale infinitely over however many servers their datacenter has without ever having to touch it...that's a miracle. It's a great idea, that unfortunately just doesn't work.
Mosso has repeatedly let us down, again and again. Their service is always going down. It's hard to find a month where one of our sites hosted on the "cloud" hasn't seen at least an hour of down time. I'd expect this from a shoddy "HOST 100 SITES FOR $2.99/mo!!" host, but not from someone charging a base rate of $100/mo. Here's what it boils down to: you're paying Mosso a lot of money for the privilege of beta testing their cloud architecture. Great business model.
And while Rackspace is known for fanatical support, the Rackspace Cloud is known by us for support that is fanatical about ignoring or avoiding the issues plaguing them on a week-to-week basis. Questions go unanswered, support requests ignored, etc etc.
So all in all, it's been a terrible experience. And yes, we have been using them for more than a month...a little over a year now. Yes, we stuck it out and payed outlandish hosting rates for horrible service. Why? Because I really do wish it worked. I wish I could put a site on it and have it be up 100% of the time. That's the point of a cloud, no? To have >= 99.999% uptime? I really wish I could put a site on there and let it scale with demand as it grew without ever having to touch it - and I can do this - but the price is my site goes down for long periods of time at short intervals (oh, plus the $100/mo). We tried to give them the benefit of the doubt, and tried to believe them every time they told us that this was the last downtime they'd be having (yes, we heard it a lot). I just can't lie to myself any more though. Mosso sucks.
So please save yourself some time and realize that it's too good to be true. The Rackspace Cloud is the most real and cool cloud hosting you'll ever see, but as far as I'm concerned they are still alpha-testing it, and your site WILL go down. Want hosting that scales automatically, is zero customer maintenance, always up, and has amazing support? You won't find it anywhere.
Mosso comes close, but they just can't get it right. Save your money and learn how to scale on a good VPS provider.