Miranda is AWESOME. I tried basically all alternative IMs, including Trillian, and Miranda is the best by far, IMHO. Trillian is nice, but is fine tuned for so many functions I don't need that it's not practical to use for me. It's also a case of a jack of all trades, Trillian does a whole lot of things, but I found many individual features to be lacking - for instance, the IRC client kinda sucks (again, IMHO).
Miranda on the other hand has formed to be exactly what I was looking for - it imported everything in my ICQ database, including my IM history of the past years (that was really a major factor to me), it's highly customisable out of the box and by the way of plugins, it doesn't do more or less than it's supposed to do, and last but not least, it's extremely lightweight, using about half the amount of RAM ICQ 2001 used and starting it up happens virtually instantly instead of taking like 10 seconds.