I find GMail totally unusable - the web site UI that is. It's fine for my wife and friends who aren't really computer savvy, but I can't imagine that anyone else finds it that useful overall.

The rules, as Tom mentioned, are severely limiting. I can't imagine not using a local client with an industry-standard server. I just started using Sparrow for iOS and so far it's about 100x better than the default iOS Mail app (which I've always disliked immensely).