I have some experience with tenUK (to whom Centurion UK were outsourcing some concierge duties recently). I can say from my experience that they were probably the most inept and hopeless company I have ever used in the service industry. Example: I moved house and need to have someone unpack and install a new Mac G5 computer. I called tenUK and explained the situation and was told someone would get back to me within 24 hours. 2 weeks later (seriously) I got a 1 page fax from them with a list of yellow pages computer engineers!
Have heard mixed things about Quintessentially. I believe if you pay for one of the higher levels of membership you get a much better service than the standard entry-level fee gets you. No surprise there then.
I generally have been very happy with Centurion concierge over the last 3 or 4 years and wouldn't consider moving from them.