BeeJive is a solid IM client for the iPhone and has been a favorite of staff members since its original release. Suffice it to say, there were a few of us who were more than a little happy when the iPad version was released today.
If you've used the iPhone sibling, BeeJive won't be that different to you. You can send files and photos from the app itself, though you can only select from a photo album for the latter. You can record audio messages and share chats with other people through e-mail, customize the wallpaper and toggle sounds among default, Yahoo, AIM or MSN. Emoticons still work, though the ability to use the /me emote is absent. You can also join chatrooms, that feature in and of itself setting BeeJive above its competitors. BeeJive still supports push notification and upon first launch will ask you if you want to enable them -- just make sure not to be alerted for each message if you are in a chatroom!