They are allright, the bottom tap ones are better as they use less gas, top taps are CO2 Hungry.
I do find that the o-ring on the tube bit that goes in the back of the tap jumps out of its groove quite often when it assembled, if missed you just get a pint of fob as the top pressure leaks out with the beer fobbing things up.
I have had a lot of trouble with the s30 valves leaking, (sorted easily as they were leaking at the valve lid connection, some home made silcone washers sorted it.
O and if the top sticks they are an absolute bstard to get off.
There were probs with New King Kegs by the way, not second hand ones.
I get on with cheapo Youngs kegs from wilkos better, 18quid a pop I don't mind chucking them when they get tired, & I only use em as secondary vessels, (intermediate conditioning tanks) before transferring to cornies now anyway.
The Cornie is the KING KEG in my opionion.
Nothing wrong with King kegs though if you know the issues with them they are easy to get on with.
UP