I'm still learning the in's and out's of character rigging in Maya (prior rigging experience in 3ds max), but I've found that MEL is really only useful in setting up rigs not animating them. As Mark's already mentioned, expressions and set driven keys are more important in the actual animating of rigs. However, you can use MEL to setup frequently used rigging configurations and scripts.

Two guys I'm currently impressed with are Jason Schleiffer of Weta Digital and John Lally of Insomniac games.

Jason Schleiffer does some amazing rigging work for the Lord of the Rings movies. He has a couple of character rigging DVDs available via the Alias|Wavefront store. I haven't seen them myself, but I have heard nothing but good things about them. He frequents the Character Rigging forum over at cgtalk.com where he contributes some great information and advice in various threads.

Also, the latest issue of Game Developer Magazine (Feb 03) has a great article with John Lally about the character setup pipeline for the Insomniac's PS2 game "Ratchet and Clank". The art team used MEL heavily for automating rigging and setup for about 50+ characters. A couple of neat things they do there is using custom MEL scripts to automatically setup animation controls and generate set driven keys for facial animation. There isn't a lot of details on how they did stuff, but there is just enough of an overview to give you a good idea of what they did and to provide you with some inspiration on your own work.

If you're wiling to share, you should post your rigs in one of the forums here (Animation would be my first choice) to get feedback on it. I know I would like to see how you work, and I'm sure others here, especially some of the newbies, would really appreciate the opportunity to learn from your experience.