In general yes they're great, but when we used Rackspace, we used their proprietary garbage Microsoft Exchange product. That is probably what the GP was talking about. It is complete and utter garbage. It constantly loses email. After switching to running our own server (a ten year-old Dell with CentOS, Postfix, SquirrelMal, etc., all pretty easy to setup and all free), the amount of mail from customers more than tripled, and we had to hire new people. It saved our business. Because Microsoft is so embarrassed by that Exchange product, they can't release source code so Rackspace can't fix any of the problems. Exchange is a nightmare, but trying to do it at the scale of Rackspace is hell. There is a reason, for example, the forty person team at Microsoft I worked for from 2002-2007 had over $200k worth of hardware to run mail very poorly. We spent about $6k per user in just hardware! When you overspec hardware by that much, Exchange doesn't lose email as often, but even that massive kit would lose messages if someone attached a file sent to the entire team. Then Exchange would thrash for several minutes and lose all other incoming mail.
I managed exchange 2007 for 500 users and we had about $14K of hardware, including the replicated Exchange server in the remote data center (but not including the AD servers and the tape backup hardware). We lost the primary site a few times due to power failure, and we had a RAID controller failure in the remote node that brought it down, and we never lost any email or had any significant unscheduled downtime. We did have to restore a few deleted employee mailboxes from backup tape due to a lawsuit, but that wasn't a problem either. It was not trivial to set it up properly, but we paid a consulting company to come in for a day and validate our configuration.
If MS spent $6K per person on hardware, it's because they wanted to, not because they had to, we did it for $33/user in hardware costs.
I left the company as we were setting up the 2010 servers on brand new hardware (virtualized on VMWare, so it's hard to pin down the hardware costs). I'm no fan of Exchange, I think it's too difficult to set up properly and requires more hardware than it should, but when set up properly, it does run well. Paying professional services fees was well worth it to make sure we had it set up correctly.