@robinbetts:
The reproofing gubbins 9nikwax, etc) doesn't reproof the membrane, just the DWR (durable water repellency) of the face fabric. If the membrane has gone flaky, it's not going to keep water out.
What usually happens before that is that the outer fabric loses it's water repellency, and then 'wets out' making it less than breathable. This prevents your sweat from escaping through the fabric, making your expensive 'breathable' jacket into a plastic bag with zips. This often happens when the garment is dirty and the water sticks to the dirt and that's why it's often a good idea to wash the jacket first before restoring the repellency.
Will it ever be factory fresh, like new? No, but it will almost always be a hell of a lot better than a dirty waterproof that gets wet and stays wet!
EDIT: the only difference between some types of re-proofer is what style of garment they're designed for, so spray on nikway is better for lined garments where you WANT the liner to wick sweat, (and not be waterproof and therefore repelling the sweat back towrads you!) as opposed to a wash-in solution.