The Pro's: Exactly as Peter T. says above; plus in jams, you avoid arguing about who is in correct tune (and the hammered dulcimer or autoharp player can't demand that everyone tune to him/her); and you can wander from one jam to another and usually not have to retune.

The Cons: 1) An acoustic guitar is never in completely correct tune - you could build one to be completely correct in one particular key, but then it would be further off in all the others - and the tuner is too stupid to make the necessary adjustments, so you always need to mess with one or two strings after the tuner says everything's fine.

2) Using one tends to cause your ability to tune without one to atrophy (this is only a concern if you ever had any such ability, so it doesn't bother me too much).

I'm a believer.

Peter.