What are advanced features of other clients? I've used the free version for a long time and would have gladly paid for it, had support not ended over a decade ago, leading to compatibility issues in Windows 7. I've tried several other free clients and some of the top rated have no GUI, none of them will play a sound on activity, many won't even flash on activity, some won't handle multiple simultaneous connections, and some also can't handle automatically logging in to a server (even when they claim to). It's 2016, we have multi-core processors, and yet SimpleMU, from 2001, seems to be the best thing out there if you're still on Win XP. Otherwise, you're dead in the water.
I still use SimpleMu in Windows 10. So long as you run it with admin rights in compatibility mode for XP Service Pack 3, it functions perfectly in higher versions of Windows.
David
