Ian,
I mentioned this in Mac's thread about accessories. I prefer the screw clamps to the quick-action ones, for various reasons:
1. I've heard the quick-action clamps described as 'one-handed' clamps, which sounds good but in reality I don't find them to be any easier to operate one-handed than the screw clamps.
2. Movement - when the quick clamps are closed, there is some slight lateral movement in the pad. Not always a problem, but if you're clamping a glue-up it can sometimes cause the work to move slightly. You don't get this with the screw clamps.
3. Controllability - I find it very easy to judge the clamping pressure with screw clamps - not so with the quick clamps.
4. Price - Current list price on the screw clamps (489570) is GBP23.05 ex VAT per pair, versus the quick-clamps (491594) at GBP32.15 each. This makes each screw clamp around 64% cheaper than the quick clamp!