Eddie, it makes you look like a pro when the guy paying you is standing there and you look like you know what you are doing when its so smooth and uniform!
Can't comment on difference between manual trans and PowerShift as I not used one with a manual trans. I do use it like mentioned in another post with my tiller, get the tiller running, drop it then go. It's also great on the F series and newer model as the shifter is on the right fender and provides for a step through frame.
Whether it's the BEST feature Yanmar offers, I'm not sure as they've had a lot of real nice options. I use a really like the UFO controls. They also had at one time a quick steering (don't know if that is the official term) option that permits the tractor to operate as a front wheel only drive. Improves turning radius by about a third. Great for those tight places. Not intended for heavy work loads but when manouvering in tight spots, it's great. Oh, and I like the creep range as a separate set of gears as it effectively doubles the number of gears.