I have them on 2 of my guitars.
The advantages are:
1) they look great
2) hold tune very well
The drawback are:
1) they weigh in at more than 200 grams x 6
2) 40:1 gear ratio is not what you expect it to be (It's direct drive, no gears inside). in reality, it still feels like your regular 12:1, my other Planet Waves tuners with 18:1 normal gear ratio (designed by Ned Steinberger too, BTW) feel a lot more precise.
3) they change the sound (add more lows), the string tension (it becomes lower) and the distance between the nut and the tuner itself (the notes that ring behind the headstock change)
The last one is not a drawback really, it is just something to have in mind.