Over the years I've spent around £5,000 with 123-reg, and I used to recommend them, but not any more.
Some of the problems I've had in the last couple of years.
There have been a couple of major outages with their DNS system, leaving sites unavailable for long periods of time.
On at least three occasions, domains that were set to auto renew, with payment details all up to date, were not renewed. This is despite 123-reg sending advance notice that they would be renewed. On each occasion I only found out after receiving an email from Nominet telling me the domain had expired.
On one occasion they took the money for an automatic two year domain renewal twice. Then they refunded one of the payments. But they only renewed the domain for one year and I had to contact them so they would give me what I'd paid for.
Four automatic renewals in the space of two days had the money taken, and I received emails from 123-reg telling me they'd been renewed. Then I received an email from Nominet telling me they'd expired. I had to contact 123-reg to get this sorted out.
When Talking Balls moved to a new server in December I changed the DNS settings in the 123-reg control panel to point to the new IP address. The control panel told me this had been done, but 123-reg didn't actually change it on their servers, which made the site unavailable. I fixed this by moving talkingballs.co.uk away from 123-reg.
I use their email forwarding on some domains. Sometimes emails that are sent to me via these domains don't arrive. I suspect this is due to them being flagged as spam. There's no way to turn off the spam filter, and no way of seeing which emails have been flagged as spam.