All our cisco network gear is currently on IOS 12.4. I've asked around locally, and everyone else I've talked to is using 12.something as well. GNS3 uses 12, and has no plans to include 15.
On the downloads site Cisco is pushing IOS 15.
What's the thinking on 15? Or is it better to just keep current within IOS 12?
Or is there more to the issue than simply a version change?