Another vote for UltraVNC Single Click. Some of things I like about it are it's small footprint (a single 170KB EXE file), and the ability to define multiple host sites (i.e. I can support a customer from home, work, etc).

As for performance and quality - it's not that bad. I don't think it is as good as GotoMyPC (which I also use regularly), however they both have their pros and cons.

If it was a single site I needed to support regularly for long durations, I would use GotoMyPC or one of the other paid for services. If it is a case of the occasion get in and get out - then UltraVNC SingleClick is usually better.