In reply to crayefish:
I've used them and found them pretty handy. As far as falling off goes, unless you're on a bouldering wall. they need to be attached to you in some way, and you can't use them for lead climbing, so I always attached mine to the rope, above the knot. If you fall and the tool remains attached to the hold it's simple enough to retrieve usually.
I found them a useful training aid. But I did end up with a painful lump in the palms of my hands that someone told me was from damage to the tendon sheath, so I'd be careful of overdoing it early on. What they'll develop is forearm stamina, plus the habit of keeping the tool still once it's placed and moving your body around it. - also because it lengthens your reach, your CoG is different, in relation to your attachment to the climbing surface - someone else will be able to phrase that better. I certainly found mine good value, but my local wall decided they were hazardous or something and banned them, so I gave them away, but I'd certainly recommend them to anyone who has a wall nearby that permits their use