Here's one guy's perspective: (Note: I'm a thin-blooded Southerner who needs extra insulation year-round). I do the great majority of my fishing in cold rivers or cool lakes and saltwater, year-round. At least two-thirds of the time, I'm wearing 5 mm neoprenes, which, worn over thermal underdrawers and heavy wool trousers, keep even me comfortable in mid-winter steelhead streams. For spring/early summer streams and float-tubing, I switch to 3.5 mm neoprenes. (Both neoprenes are Cabela's, inexpensive but entirely adequate.) For summer-mild streams, I enjoy the light weight and comfort of breathable stockingfoot waders.