So why are people switching? Well, the average non-technical person at best has one IM account and one client -- and it's usually one of Yahoo, MSN, or AOL. But more technical users that communicate with a lot of people over IM are forced into using multi-protocol clients. While these clients can talk to all the systems, they often don't work well with basic functions like file transfer.
And this is where Skype comes in. It's a good IM client, it has wide-spread usage among the people that are likely to be using IM, and it has fully encrypted communications. The file transfers work really well (I've never had one fail, even behind firewalls), and you can even transfer contacts to someone else.
Oh right....AND it does voice, including conferences. Gersham had this to say (over Skype, of course):
I find the usage pattern for me with a bunch of people I use with it is to start IM'ing with them, then switch to voice and then back to IM.
This is exactly how I use Skype. It allows me to mix voice and IM as needed. When I'm busy, I can IM and multitask. When I need to switch to a focused, information rich conversation, I can make a voice call.
Now I'm feeling like a Skype commercial. Well, last tip for this ad: change the default double-click behaviour to send an IM instead of initiate a voice call...your Skype conversion is complete.