I had a good experience with Black Diamond (for a Bibler tent) on getting replacement pole sections when some bent in a storm.

I have two Hillebergs but they're so good I've not had to avail myself of their customer service :-) However, I hear it's good as well - Petra Hilleberg responded directly to my inquiries when buying the tents, and another of their staff answered the phone on the first ring and provided helpful answers; they also will allow you to set up a tent in your house or yard and send it back if the model doesn't suit you.

Never dealt with them on tents, but MSR has been great about service and fixes on other items (stoves and snowshoes in my case); I imagine it's an overall approach that applies to their tent line as well.