You cant go past kickboxing for a good cardio workout. Try doing a 2 minute round on a 5 foot bag straight boxing, then do one with kicks as well. You will notice the difference in a big way. The kicks may be a problem for some of the bigger guys though due to lack of flexability.
To protect your wrists just make sure you wear strapping under your gloves and make sure it goes up your wrist enough to stop them from flexing too much. I wouldnt go near a heavy bag without strapping, especially once you learn how to load up a punch properly with your hips etc. There are also a lot of delicate bones in your knuckles that you can damage and they are not easy to fix once broken.
A few boxing classes will get your technique right, and then you can use it forever. Bad technique = injuries.
As for brands, I agree, Everlast bags dont last long at all. I use Bodan and they last for a long time. I have kicked the crap out of mine and it just keeps going.
IMO there is no better workout than hitting a heavy bag for cardio. I also reckon it helps shape your muscles better than anything as well. Add the stress realease it can give you and of course, knowing how to take somones head off should you get caught in a situation - would be mad not to take it up.