I regularly do cold water swims in spring and fall months, and wholeheartedly agree with the amazing feeling after a good 10-20 minute dip.

I find the first few minutes of the swim are the hardest, as your body is adjusting to the cold, surface blood vessels are constricting, etc. The next 5-15 minutes are fantastic! I don't usually push it much beyond that.

But any discussion of the benefits of cold water swimming also needs to highlight the risk. If you're thinking of trying it out, make sure you're acutely aware of the temperatures you're dealing with, how long you're planning on being in the water, and that you understand the body's reflexes to cold water submersion. Here's a good resource covering these points: http://seagrant.umn.edu/coastal_communities/hypothermia