I'm using the same slapstrap I bought when I was initially certified, which was in 1985! I have had it on three masks, and I will not dive without it. I have the velcro ends indexed to what is comfortable for my head, and it works perfectly. When I got a new mask (SeaVision, with bifocal correction) about two years ago, I got a new slapstrap and fitted it to my new mask (in lieu of the "factory" neoprene strap that came with the SeaVision mask). For some reason, I could never get the mask to fit correctly, regardless of a number of adjustments to the new slapstrap. In desparation I went back to my 20 year old original slapstrap, and haven't had a problem since.
I highly recommend one over a standard neoprene, I feel so much more comfortable in mine.