10 years of Irssi use and I switched to WeeChat last weekend
I started using Irssi almost 10 years ago when I first started trolling wandering around the world of IRC. My main use is to run it in a screen and stay connected all the time. To chat I'll just ssh into the server and reconnect to the screen. Generally I leave my terminal open, stretched across the top of my monitor so it's really wide but only about 8 or 10 lines high. This way I can keep IRC visible all day long and respond quickly to questions.
I've customized irssi with custom highlights and commands and all has been well, save one thing: bug 310 - vertical splits. Irssi can't do vertical window splits and with the trends giving us widescreen monitors, horizontal splits aren't really useful to me. The bug has been open since 2005 and I've all but given up on it.
Then the other day Chris McDonald claimed the unfortunately named WeeChat was superior to Irssi. I was all ready to defend Irssi's honor but I read through the documentation and WeeChat was pretty compelling - most notably its support for vertical splits!
So, long story short, I switched last week and it's awesome. I don't miss Irssi at all and in fact WeeChat offers me things I never even knew I'd want (like per-buffer history when hitting the up-arrow). I've customized it so it essentially looks like Irssi (no nickname list, etc.) and I've added a lot of aliases to make it easier for me to use (mostly vim keys). If you're an Irssi user you owe it to yourself to at least read their docs and see what you think.