The main issues people have with the native AIM app:
- Message delivery lag of up to two minutes.
- No buddy icons / avatars.
- No status / away messages.
- Buddy list doesn't always automatically update.
- Can sometimes log off without notice.
Other than that, when it works, it does really good. Since it uses push data, there isn't much battery drain. If you don't mind the 2004 interface, it's worth the price.
There are lots of other choices out there as well. It all depends on how you use it as to what you'll get out of it.
Free IM apps:
- IM+ Lite
From what I have seen, the free apps don't have any type of push technology included, so they may use more battery power. Of the paid apps, I think Trillian (which is also the cheapest) probably has the best background option (doesn't need to be running while still logged in), followed by IM+ (which is the most expensive) and their use of e-mail for push messaging. BeeJive has to be running inthe background for push services to work, but it probably has one of the nicer interfaces.