The last overlay I did was on a trashed Gorgar playfield. Actually the entire game was a basket case, and it was an exercise in bringing it back to playable and presentable condition.
The overlays back then had the inserts "colored" in the overlay as opposed to the clear inserts now available. Installing the overlay wasn't exceptionally difficult, but trying to line up ALL the cutouts, inserts, etc. was an exercise in patience, and still probably about an 8 out of 10 when done. I've never seen one that was spot on perfect in this regard. So figure you'll be cheating and compromising somewhere when installing.
The colored inserts were the biggest complaint as far as cosmetics, along with a loss of detail in the artwork. You lost the "jewel" like appearance with the old overlays. I suspect that the method above would preserve the effect to the best possible result, but seems like a TON of extra work. . Not sure how the clear will show the inserts through.. probably acceptable would be my guess.
As far as playability, the results were very good. It was VERY smooth, VERY quiet, and somewhat "soft" feeling... the few mm of vinyl thickness should make this somewhat obvious, but it is a much softer substrate than the ink printed plywood of an original.
Lastly, the durability in my experience was very good as well. No issues with waxing, loss or artwork, scratching, peeling, or other adverse affects. I had the game for several years and played it regularly. I had heard of others who clearcoated OVER an overlay and had poor results. I wouldn't suggest trying it due to the thickness and give of the overlay itself. I'd say you're asking for cracked and peeled clear if you do.
If you just want a better looking game that plays well, then go for it. Don't spend extra time and effort to make it look better, since it's still going to look unoriginal. Certainly better than peeling paint though!