I used them before but back in 1999 when I was renewing 3 domains for 3 years at a time and their price was better then my other host (the company verisign bought, networksolutions.com) and register.com offerred free transfer & a better price.
Based on my experience I'd never use them again. The price is highly inflated, and although my comparison isn't fair because it was 2002 when I last used them, the usability of godaddy.com and namecheap.com was much better IMO then register.com. So----I'd never go back to register.com unless they beat the prices of NAMECHEAP or GODADDY, and even then that's a big IF.
Just my $.02.