Thought I might be able to offer some insight on my experience of 123-reg.
Domains are domains in my mind,it doesnt matter where you register them provided that the people you register them with has a decent system for renewals etc and offer you the functionality you are looking for. e.g ability to change the nameservers/
DNS.
For many years I used 123-reg without problem - you never really need support on a domain name its always on the hosting (dont get hosting from them).
Having said all that.... approximately 6 months ago I logged onto the 123-reg site to renew one of my customers domains. I renewed it successfully 10 days before it was due to expire.
15 days later I received an email from nominet saying that I had not renewed my domain name and that the domain would be returned to the pool. I then attempted to contact 123-reg to resolve the problem as the status of the domain was showing as "renewal overdue". It took me approximately 1 hour to get through to a customer service representative who resolved the problem in under 10 minutes. They had charged my card but not submitted the renewal request to nominet (the people who look after ALL the .uk's)
I then thought "it's a one off, ive never had this before - I will continue to use them"
However I had the same thing happen this month for 2 domains. I have been on hold for 3 hours to tech support only to get a recorded message saying they were having problems with billing and to please bare with them whilst they tried to resolve the problem. This is costing me time and money trying to sort out so I will consequently be switching away from 123-reg.
If anyone knows of a decent firm who do domain reg (don't tell me about the big ones please, I know all them)
I hope this helps, I have tried to give as fair a view as possible from my 3 years of being a customer with them with approximately 60 domains.
Regards,
Dewetha