Have you used them?
Yes, I have for the last four years, have three domains, and all they do is keep adding features and capacity without raising my rates. As a matter of fact, I was just rebated half my semi-annual fee for some reason. It costs me $30 every six months for an ungodly amount of storage, bandwidth, emails (including IMAP), subdomains, tools & utilities, plugins, support for any of my stupid questions (that typically get answered within an hour or so), and anything else I could think of. In fact, I just switched from MS to Linux hosting, and it took all of ten minutes, no charge, even though the Linux packages contain more functionality.
A couple of months ago, there was a muck-up on the regular billing (my fault) and my account was locked. Mind you, all my domains remained completely active & functional during this unpaid period. I cleared up the billing, sent an email to Support letting them know (and would they please unlock me), and it was done in about five minutes.
Honestly, there is nothing I could conceivably want or need that isn't offered by them already--including customer service, and when I check my admin panel periodically, another new feature or tool's been added.
I periodically compare them with the other providers, and see absolutely no reason to switch. As far as complaints and problems, I see more about GoDaddy on this forum than I have ever seen about 1&1.
It does, however, lack bouncing boob ads.