Tried a cock pump once or twice but didn't like it. The sensation is very strange and it gives you a weird looking errection, all veiny but in a strange way. Had a strange effect on the nerve endings as well, not unpleasant, just different. As the blood isn't flowing as such, your cock goes a deep purple colour
Think cock pumps were invented for blokes that can't get a stiffy. You pump it up, pop a chick ring on the base of your dick and you have a stiff cock of sorts to fuck with. I'm not an expert, but I guess there is the risk of fucking up the vascular system in your cock if you do it too much or for too long.
It wasn't for me, but did get me back on the road to proper stiffies after a frustrating period of floppy cock syndrome. Maybe cock pumps work better if you have a natural errection and then pump that. Fuck knows, my cock works fine now and I won't be using pumps or rings again.
Whatever floats yer boat though.....