i bought a set of these. I haven't used them a great deal, but from what I have they seem ok to me. The plastic covers fall off from time to time, but are easily put on providing you don't loose them. The pins are attached to the body by a piece of wire which has detached itself on almost all of the clamps. It is not a problem, again as long as you dont lose the pins.
In my experience, the clamps hold well. But as I say I haven't used them a great deal since buying them a year ago.
If I was buying again, I would probably try and get some pipe clamps and pipe instead. This is because having too long sash clamps is as bad as too short sometimes, and it would enable me to have a lot more sizes than I can afford in sashes. I got the rutlands ones on offer and for a tenner a piece, to my doorstep, i think that they are worth the money. I dont doubt that they are vastly inferior to the likes of Beesey, but I couldnt justify much more money at the time for a hobby.
If they are going to be in daily use etc, then perhaps check the advice of others. You may get better value for money from beeseys or other things suited to years of professional use.