For the better part of the last two years, Yahoo! Messenger has worked like crap for me. Basically, I talk to three people with it: Stephanie, Catherine and recently the designer I hired to do some work for me.
For the longest time, Steph would never show up in my buddy list. (I jokingly blame our divorce on this. :)) Then one day, poof, there she was every time. Then last week, she's gone again.
Then today, I get an IM from Jenna (the designer) with some questions, and while in my list, she appears not to be online. What the hell? Yahoo sucks. At least I'm using third-party clients instead of their big bloated piece of crap.