But if you send a message they won't see your message until you're both online again. So if you sign off before you sign on, it won't send until both clients are online again (could be days).

If you use multiple computers it gets worse - if you send the message from your laptop it won't go through until the laptop and your friend are online again.

Its not a huge problem with a 1 on 1 chat, but with groups its frustrating.

We benefit greatly from having the chat room as sort of a canonical log of what's happening. Its nice to be able to leave a message for coworkers before you sign off for the night, and know that everyone will see it. With Skype we didn't have that guarantee.