I have tried X-IRC (i think that's the name) when i first purchased my mbp, but I then tried X-Chat Aqua and have been using that ever since. Very nice program and it also is fairly streamlined imo. Has some pretty nice features, and stable.
I like them both, but for different reasons. Colloquy is more "OSeXy" while X-Chat I think is a more barebones, no-frills approach. X-Chat would be more similar to mIRC, I think.