DD's recent thread on his search for a stopwatch holder got me thinking, and once the coal smoke had cleared, I found myself looking (predictably) on eBay. There sure are a lot of vintage stopwatches for sale! My problem (or at least the most salient) is that I don't know the first thing about these mechanical marvels.

What should I look for, what should I avoid, what would be a good example for a cyclist to have as part of his training kit?