ICQ is horrible. I have no idea why folks haven't started migrating off of it more than they have. While the idea of offline messages is great, as-is it's worse than not even implementing them at all.
Also it's inability to receive "duplicate" messages from multiple locations (e.g. workstation, laptop at home, etc.) is *really* annoying as well. I don't want to have to remote desktop in just to talk on ICQ.
AIM these days is my preferred IM network. It Just Works(tm). Yes, no offline message capability, but that is what e-mail is for. If I send a message, I can be reasonable sure the recipient did indeed receive it. ICQ, not so much.
It's no longer 1998
Time to make a switch in my opinion.