We currently co-locate from a Qwest CyberCenter on the East Coast. It is super clean. We have never had a problem with this data center at all. I house over 20 servers there just for our website alone.
You are correct, it might be better to co-locate somewhere carrier neutral, but we have a nice relationship with Qwest and have never had an issue. We have had 0 downtime that was not scheduled by us for our own upgrades and maintenance.
We were once colocated at a facility that had many providers, and it was a total horror story. It was always he said, said.. One blaming the local loop problems and others blaming the bandwidth provider. Now I know this is not the same for everyone as there are some wonderful places out there, but so far with Qwest we have had no issues and plan to stay there for now.
We have been there about 2 yrs. We have 4 cages dedicated to us. Right now they are pretty flexible with pricing as they want people's business. I have hit them up for things that I could not get before that I am getting now.