I'm not a fan of adding GC to C. I've hard my fair share of stress caused by GC issues. It's great 99% of the time but when you run into performance issues caused by the GC it becomes a very very leaky abstraction.
I haven't had any problem with performance with the Boehm GC personally, though for what I've used C for is not-real-time video processing stuff. I found I typically got better throughput using Boehm than I did when I was manually managing my memory, but for what I was using it for, a small pause wasn't really a problem as long as the throughput wasn't really affected.