Shattered Isles started with coffeeMUD and still uses a heavily modified version. When you get into customizing it outside of the easy to use tools provided, you'll find yourself gutting a lot that may not apply to your style.

We broke off at version 5.62, upgrading at that point only because of the addition of accounts for characters. Stability was always the concern going in, but it's never once been an issue that wasn't self inflicted.

I don't know what your measure of success is, but we run around 20 characters during primetime hours, and 75 active accounts. It's definitely possible to get a coffeeMUD game off the ground. You may want to search for the coffeeMUD yahoo group. Great support there.