Cons are restricted air flow on most of them and they take a big getting used to as the pipe extends a bit further into the mouth due to the design. Might not bother you but it took some getting used to for me.
Pros, easier to clench with the little lip, if you have tongue bite you can smoke them without irritating the tongue as much. Good if you are clencher.
To open up the lip, just take a drill bit and gently rotate it around the hole. The reduced air flow is from the hole, so when its opened up a bit, it will smoke fine. Some people take a needle file and then smooth out the hole. Just be careful you do not drill thru the bit and make a hole on the bottom side.
I used to not like the p-lip but after smoking them for awhile, it makes no difference to me if its p-lip or fishtail. It's just part of the Peterson experience.