I use a Montane featherlite and found it only of use in light showers, anything more and it leaks like a sieve, but it does keep the warmth in.
But my perception of it changed last year during "Season of the Mists" where as Colin will testify it heaved it down for hours. I was soaked to the skin before I set off, but at some point, things changed, I became aware that I had dried out and water was beading off the jacket and it itself was bone dry.
All I can think of was that I gave it a good waterproofing with Granger waterproofer and cleaner about a week or two before, but as I do not have a tumble dryer, I could not do the treatment correctly as the jacket needs to be heated up in a dryer to finish the treatment off after applying the waterproofer. But whilst riding, SOTM, my body heat once I got going may have activated the waterproofer properly and it started working. Its only a theory, as one minute you have a jacket that leaks like a sieve, and about a couple of hours later, waters beading off it for some unknown reason. After 5 hours of torrential rain before it stopped, the top part of my body was bone dry, bottom half was soaked though.
As a rule though if its warm, I just wear a cheap short sleeved replica jersey from Hong Kong preferably made from polyester, and not an expensive coolmax material such as Endura as I find that polyester (even though it may be labelled coolmax) keeps the heat in better, in good quality coolmax, it can get a bit cold in the rain, and I never wear a waterproof unless the conditions are very bad. Usually within about 15 minutes of it stopping raining, I'm dry again.