Reputation tells me that someone has posted a lot of helpful answers ... the best indicator is to see if someone is highly active in your tag AND has a high reputation.

Reputation, alone is just a neat way for SO to say thanks to people who participate. Badges are another matter, looking how active someone is in a given tag brings it all together.

I'm highly active in Linux .. so if I answer a .NET question, someone with a reputation of 1 may (and probably does) know MUCH better than I do.