I've never been a customer, but I've done project work for a company who uses them for all their comms. They messed up a config change one Saturday which lost us half a day's service and their support wasn't up to much but I think it was one guy on his own, but otherwise they were reasonably happy with them.
I have also heard good reports from a friend who resells their comms and said they are very good to deal with and getting new links in quickly. With OpenReach I've had projects take between 2 weeks and 8 months (and counting) to go in, and you never know how long it'll take.
Their SLA's look good on paper but I think they would be seriously struggling to meet the repair times in the event of an equipment failure. I've also had my fingers burned with wireless links in the past.
Would I use them for comms to a small branch office? Probably. Would I trust them with a large HQ for an online business? That would be fibre every time.