I have one, I use it dry.
I started PE at the beginning of August, I used my hand to dry jelq everyday for 2 weeks but my girth actually shrunk slightly.
I Started using the jelq device at 50-70% erect with 3-5 second strokes for 15 minutes every other day and my shaft feels noticably thicker after about 3 weeks.
I never liked the mess of normal wet jelquing so the jelq device is perfect for me however the rollers don't roll until the skin is stretched (I'm uncut) so it isn't very comfortable and pubes sometimes get caught up in the rollers which can hurt.
If you can afford it, get one but it probably isn't any better than using your hands overall.