I have a pair of these and, when I remember to use them, they work well. Because they take a bit more time to set up and use, I more often reach for my Bessie's. However, these do help to hold a panel flat and, at least partially, minimize the need for additional cauls. They do not apply as much pressure (IMHO) as a caul might so they might not be as effective. Also, I have no idea if the clamping pressure is comparable with other clamps.
If you can get them at a good price I say go for it. They will just be one more tool in your clamp arsenal....and you can never have too many clamps.