I use Rackspace Cloud for my server and I can tell you they are a fairly good host (excellent customer support, superb uptime). Setting up a cloud instance with them is a snap, and happens fairly quickly; also, making a backup is very very easy.
However, it's not all fun and games - their control panel is slow, is not quite as powerful as I've found from some other hosts, they do not (yet) have a programmatic DNS api (and they handle DNS slightly oddly) and their out-of-band console is rather weak.
You may be interested at also taking a look at Linode; we use Linode within our company for individual applications (for example, we have one cloud instance for our task management system). They hold up very well, although Linode did have some horrific downtime in the last couple of months (they should've fixed the source of this, though); I discovered that they're pretty slow to update their website when such a crisis happens, but their staff and the community in their IRC channel were great and very helpful. In terms of their control panel, my only primary complaint is a lack of granularity in the bandwidth reporting (though I don't touch the control panel often, so I'm quite possibly using it wrong)
If you're after a performance comparison of various hosts, Eivind Uggedal seems to have done a decent comparison of Slicehost, Linode, Amazon EC2, Rackspace Cloud and PrgMr: