I've transferred a couple of my domains to NameCheap.com.  So far, I'm impressed with their user interface and their customer service.  I especially like the WhoisGuard service that they are currently offering for free, which protects your contact information.  Instead of your contact information (name, address, and e-mail address) being public to anyone, they replace it with their contact information and a randomly generated e-mail address that forwards to your own.  You can also have WhoisGuard automatically change the random contact e-mail address ever XX number of days to really cut down on the amount of spam due to e-mail harvesters.

The first domain transferred within a few days without any issue.  However, the second domain stalled at the first step of sending an accept/reject e-mail to the registered contact.  This didn't work because the WHOIS for the domain didn't have the full contact information, but a message that read, "For complete domain details go to..." with a link to the current registrar's site.  I contacted NameCheap.com's customer support and within 24 hours the domain transfer was on track again.