I've had a very similar experience with Rackspacecloud (makes sense since they're the same company as Slicehost now): Incredibly slow control panel, features sometimes work and sometimes don't, useless tech support (took 5 days to point our server to our domain because someone else had it pointed first for some reason, despite whois being obviously ours).

Meanwhile, I've been using Linode for my personal projects and it's night and day. The control panel is incredibly faster and easier to use, the prices and specs are better, and the community/documentation is noticeably more intelligent and better-equipped.