The refill kits are okay, but only to top-up a system really, ideally you need to vac out the gas hold it to check for leaks, you can't really do that with the kits, and the cheap ones don't even have a pressure gauge with them, and you REALLY need that
I've taken LOADS of our vehicles to Kwik Fit to have the air con redone, they check the temprature first and then do the work, if the system leaks, you don't pay, if it doesn't make it colder, you don't pay (and it's colder by a number of degrees, not just 1 or 2). I've driven away a few times for free, it's one of those "extras" that they do.
Oh yes, they don't do the "screw thread" (R12) type connectors (like I have on my SVX), but anything after about 94/95 you should be okay with.
I always go there for brake checks, they strip the entire system down and tell me exactly what it needs.... I then go and do it with no awkward bolts to remove as they'd done them all in the free brake check