I've done many PE sets in my time,but I've never seen or heard of adhesive PE sets.Just to be clear,I mainly am talking about 1/72nd scale aircraft PE sets.

What are they?

Are they better than painting the instruments by hand?

Do they look better then hand painting?

What are the pros and cons of you using them over hand painting?
Thank you for all of your comments and information.

There are a lot of variables here. I've done a lot of PE as well, and some of it has been "adhesive backed", especially some of the newer Eduard aircraft interior sets. What it does is it keeps you from making a mess with CA when gluing two painted parts together. That's a good idea, but frankly, I've never had that problem. Instruments seem a little better defined to me with adhesive/pre-painted PE, but really, it varies between PE sets. I still run a bead of CA along the edge of adhesive PE, because I don't trust over time, the adhesive might separate. Adhesive PE does make the job of applying PE a little easier. It really depends on your scale and the subject you are modeling. You might get a better response if you were more specific about the exact PE set you are considering.
VR, Russ