I am very impressed with Colloquy. It has the right balance of Mac integration and candy-colored beauty. I can tone it down to a Snak or Ircle level of geegaws and still have my passwords for nick servers stored in my Keychain. This may be the first IRC client to get me to break the habit of chatting in fixed-width fonts. I haven't tried silc yet, but I'm glad to have the ability in a nice GUI-enabled program.