I have both a Trend and a Kreg jig and find them invaluable for those quick and dirty jobs especially around the 'shop.
I think pocket holes are great for the utility things i build and for things which wont be seen, like kitchen cabinet carcasses. Can't be beat on joinery to finished product time!
It is of utmost importance to clamp thight, though, as mentioned.
Pro's:
Quick and easy joinery
Takes away the need to clamp while glue dries.
Con's:
Ugly to look at, even if plugged holes IMHO, although contrasting plugs may look good in some applications, and user made plugs can be made to match the material better.
Clamps needed when screwing to avoid movement of piece due to angled screws.
I think the Kreg system is the best, they have made it possible to consistently get good results easy.