I have been a Cobra dealer for a little under a year and so far my experience on my sons kart (jr.1) and my buddies kart (sr heavy) the results have been good on our local home track which has an open tire rule. My son runs some maxxis only tracks and some open tire tracks and has had success with both tires. From my experience the cobras are as fast early in the life cycle but do not last quite as long as a maxxis but the return is relative to the price. My son has run cobras that have been cut, internally prepped, no prep, and about everywhere in between and so far honestly the best program we have found was to mount them, set the stagger you want, scuff them, and then just use pre race wipe at the track thats all. this method has worked in the jr classes as well as the sr classes with success.
Tire opinions are what they are and what works for some may not work for others. In my case i was looking for a maxxis alternative to experiment with and so far they seem to work where we race. I sell them for about 60% of the cost of a set of maxxis tires and get about 70% of the life of a maxxis and so far i will take that trade off.