Great for people who want some advanced configuration!
I have been using 123-reg to host domain names for several years and have only good things to say. I have also recently purchased a VPS and an SSL certificate from them, both have been great.
I have recently switched VPS from Webfusion (their parent company) to 123-reg to take advantage of CentOS6 and cPanel at a great price. It's hosted in England which means low latency and support during working hours. Both VPS's are in the same location and uptime has been very good with only 2 small blips.
Whenever I have had any support queries I have had a clear response that has answered the question straight away. Their abuse department also looked through some evidence of portscanning. All in all a great experience so far.