Ahh, we agree on something!
I use flat matt white paint as well, it's on average about 5% less reflective than mylar and doesn't reflect as much heat. The biggest problem with mylar is, people don't use it properly - it needs to be absolutely flat to maximise its reflective properties, if it's got any crinkles in it at all, then much of its reflective advantage is lost, and mylar is so thin it's hard not to get it crinkled.
The thicker diamond pattern mylar probably is worth the money as its more reflective tha n standard mylar and doesn't crinkle anything like as much. However, you have to weigh up how much that extra 5% reflectivity is worth, for many it's simply not worth the money, particularly if you have to cover large areas with it.