Powerblocks are great.. I have the Pro Series Set. I opted for just the basic set to start and i'm looking to buy the expension set now. They are price fixed so unless you find a used set they are the same price everywhere. The mfg will not let the retailer lower the price. But, if you need anything else you can bargain on the other items.. thats what I did to get another $75 off the bench i needed.
I have seen the screw end type as well ( i forget the brand ) but they took way too long to change the weight. The powerblocks can change in just a few seconds. The screw type took longer to change than normal weights.
On a side note, the pro series have small cylender weights that go inside the handles. As you go through your normal motion these weights will shift about 1/4-1/2". Doesn't seem like much, but depending on the motion you can feel it. Also, go to a store and try them. They feel really good to me, but once you get up to the higher weights they are quite large. They are much longer in the 90+ range than a normal free weights.