My first ‘prescription’ mask was an old USD oval mask that I glued glasses lenses into. I used lenses from a pair of military-issue glasses because the front surface was nearly flat. I think I used ‘Goop’-brand glue. It worked just fine. Of course, I had to let it sit for about a week before I used it so I wouldn’t get high while wearing it…
I used it for snorkeling and part-way through my OW cert. Then (1993) I bought the salesman’s ‘low profile’ pitch and bought a USD ‘Hawaii 2’ mask w/ even-diopter lenses.
On most of my dives I’ve use disposable contact lenses w/ the flat lenses in the mask. However, in classes where I know I’ll likely be removing my mask, I put the prescription lenses back in, and don’t risk loosing contacts. It’s fairly easy to switch back and forth.
Like JimLap mentioned, when I use the prescription lenses I leave my glasses on ‘til the last moment, then put ‘em in my mask box when I put my mask on.
My prescription lenses are –6.0 and –7.0. My current contact prescription is –6.5 and –7.5. So without corrective lenses I have to have someone lead me to the water.
I can tell that I can’t see ‘quite right’ on the surface w/ my prescription lenses, but underwater it’s fine.
I also always carry an old set of glasses in my dive bag, just in case something happens to my glasses or contacts while diving. That way, at least I’ll be able to drive home.
Also, when and if I start carrying a backup mask, it'll probably have prescription lenses, on the assumption that I'll loose my contacts if my mask get's kicked off. However, here on SB there have been several people claim you can take your mask off underwater, open your eyes, and you won't loose the contacts. So far I've only tried this in extra-confined-freshwater (the bathtub) and they stayed put.
I dispose of my disposable contacts after diving, just in case they got any nasties in 'em.