Ever noticed the following with Skype: Go offline while several of your friends are online. Suspend your computer. A few hours later, resume your computer and go back online with Skype. Notice how it claims all the buddys that were online when you disconnected just came online? And a few seconds later, some of them all go offline? And several minutes later, they even come back?

Skype (well, at least version 1.3.0.53, YMMV) obviously caches which users it has seen and just assumes they are still online. When it then has actually connected to the Skype network, it notice that none of its peers has them and decides that they maybe aren't online after all. Then it actually searches them throughout the network, and when it has found them puts them back online again.

Thats what I call cheating. Other IM services are much more reliable here. And other IM services - at least ICQ and many Jabber servers, though not Google Talk - also have offline messages. I.e. you can write messages that will reach the receipient once he goes online again. Skype again cheats on these: if you send such a message, it will after some time display the notice that sending it failed. The next time both of you are online, it will actually be delivered. This can result in suprisingly old messages, and makes connecting to people much harder: for putting someone onto your list, you also have to be online at the same time.

Assuming that you are using Skype only during work hours and going offline inbetween, this makes communicating with people in different timezones a pain.

File transfers with Skype are also a game of chance: If both of you are somewhat firewalled (e.g. behin a DSL-modem-router combo), it often can't transfer directly. Instead it picks a random "super node" with large bandwith for transferring the files. However, the super nodes bandwidth isn't always as good as expected by Skype, so sometimes file transfers with Skype are awfully slow, even when your line isn't busy. MSN for example (though I do NOT suggest using MSN), is using it's own, well-powered servers to handle such file transfers (or maybe all; thats why I don't really suggest using MSN, since I've only seen it transfer my files through microsofts servers...). But usually file transfers via MSN - especially in firewalled situations - are much faster than with Skype. (A drawback of them is the short timeout)

So consider using XMPP/Jabber (= Google Talk) instead. They are more reliable, and an open, well-defined, extensible standard.