WHile the medium tank is great, I still use the mammy a lot if the situation is right. I use them a lot for Hourglass since two mammies can effectively barricade the GDI entrance from APCs and flame rushs. Plus, double fire power (vs. medium) is very useful when shooting at incoming vehicles.

For maps like islands, though, I prefer medium tanks since there's no obelisk to worry about. At least until the games been going on for a while and then a mammy rush works well, but usually Islands games are short.