Most of my chess play is broken out between OTB and some online chess; most of that is on Chessworld. (Occasionally I'll play a quick game on Yahoo.) The practice I get between the over the board and the slow games over the Internet on Chessworld are good for my development.
However, I still appreciate playing chess through the mail. I played it for a while through the USCF, but it did not have the staying power for me. The one person I have been playing for quite a while through the mail is my dad.
I have not lived in Ohio for over a decade, so playing chess through the mail is a good way for us to keep in touch. We do talk on the phone, but the short notes that we include with our moves are a good way to capture moments in time.
Unfortunately, with the exception of the two current games we are playing (we started these in 2005), I have not entered the games in electronic format. I am doing that now so that I have the option of bringing up the current position without having to play through every move in both games.