I've used them for several years.
DON'T PUT A FINISHED SHIRT IN THE DRYER.....EVER!!!! hang them up to dry.
It's true that they will begin to fade bit or slightly crack (depending on the transfer used) but they should last you quite a while if you don't put them in the dryer. It's the dryer that ruins them.
Also, don't do any design that's too large. Stick to about 1/4 of a page or slightly larger.
Follow the directions to the letter. Print on the right settings, round your corners, use something solid under the shirt when ironing and put a good amount of pressure on it (i use as much of my body weight/force as i can).
Pay special attention to edges/corners when you are ironing.
I have shirts that have lasted and still look good after over a year. I even have one from about 6 years ago that, while pretty faded, still looks ok.