As you I'm sure have figured out, I've been enthralled with this GNS3 product. But there's a few problems that I've hit:
First Problem: Every time I recommend GNS3 to someone, they get all excited until they find out they need a copy of the IOS to run it. The next question becomes, "So where to I get that?" At which point, I'm forced to mutter something about using someone's SmartNet contract. And I honestly feel bad! There's so many people that want to learn Cisco that can't get a copy of the IOS from somewhere.
Second Problem: Even if you DO get a copy of the IOS and get GNS3 set up, you end up in a, "what now?" scenario. Let me describe...When I teach Cisco classes, I always encourage people to get some lab equipment from Ebay...but I know a lot of people do this, get all this shiny, racked equipment in place and then don't know what to do with it.