My fren (now ex 3M staff) told me that for 3M, it is always a quality product. V-cool which is famous because a lot of the car franchisors use it, has many grades. I was told the one fixed by the car dealer are the cheaper ones but the buyer have to pay the premium price. If you go direct to V-cool dealer, you will get a better quality tint .. the only thing is some shops will resort to use the imitation or the also the cheaper one.
My 3M tint is good, no complaint so far. It does reflect the heat and UV as intended.