Years ago we had them managing about 50 of our servers - basically overflow work. When we needed repetitve tasks done on servers we had them do some of the work with us while our admins concentrated on other things.
We've had a good relationship with them over the years, and I still refer people to their company that are looking for server management.
You must remember that they are a REACTIVE Management company - they will only do the work that you tell them to do. If you know nothing about servers and don't know to tell them to check the logs at x,y,z you run the risk of paying for services you don't use.
If you have specific setup requirements, make sure that it's documented on EACH TICKET. They do not have a cheat sheet for their admins to look at before accessing your server and enabling phpsuexec after an Apache recompile on a server that shouldn't have it enabled. It can create quite a headache some times.
For the most part - they've been a good company for us to work with. The last time I worked with them personally was about 4 years ago.