I've been using Rackspace cloud for a couple of clients and reckon they're lagging far behind Amazon and the other providers. They've bet the farm on the OpenStack project but given the number of players involved (over a hundred!) the pace of progress is glacial. It lacks really fundamental features (no private backend network, no elastic load balancing, no [public] support for elastic IP's) and these problems make it a pain to use for all but small projects.
In fact the only reasons I can think of to recommend it right now are the generous pricing and the way their cloud instances are more VPS-ish (i.e. unlikely to lose local state) than EC2's.
Yes, they do have load balancing. But not elastic load balancing - there's no automated way to scale the size of your platform in response to load. So you end up futzing around with a bunch of scripts to do this and inevitably do it it badly.
In fact the only reasons I can think of to recommend it right now are the generous pricing and the way their cloud instances are more VPS-ish (i.e. unlikely to lose local state) than EC2's.