I've had one for a long time (a Gripmaster, to be specific). When I first got it I was learning upright, and I over-used the device so I started having wrist pain and stopped using it for several years. Recnetly, I threw it in my car and use it in short bursts while waiting at red lights, drive-thrus, etc.
I've found it helpful in the fretting hand for improving left-hand tapping and other percussive techniques, but honestly, I've found the most benefit in the other hand.
My band has several long sections of fast eigth notes where I'm using three fingers, and I often struggle if I don't keep up my practice routine. When I first put the gripmaster in my car I was pretty busy and didn't have as much practice time as I wanted, but I did practice the right hand motions on the device in the car throughout the week. When I got to rehearsal, the parts were more effortless than they ever had been in the past. I've been sold ever since.