I have tried A LOT of mascaras, but this is the best I have ever used. I have naturally blonde lashes that are getting sparser as I get older and have been loading on the mascara since I was in my early teens!
I was initially reluctant to buy this because of the price (I normally stick to under £10 for a mascara) and because it isn't waterproof (I'm a contact lens wearer and sometimes find my watery eyes leave me looking like a panda with non-waterproof mascara!) However, I'd say it was good value for money at the RRP and I haven't had problems with it running during normal wear, including in the rain (I did swim with it on once and never again - don't be confident that it is properly waterproof!)
The mascara applies really easily and one coat gives you a natural, long-lashed look, although I prefer quite a dramatic look - easily achieved with a couple of coats. The mascara doesn't clump and has a spiky ended brush to get to all the little lashes at the edge of eyes that often get missed. It definitely lengthens my lashes, keeps them separate and also curls them so my eyes look more open.
I love this mascara and would recommend it wholeheartedly!
Incidentally, if you're certain you do need something waterproof and still fabulous, Maybelline's the Falsies Volum' Express Waterproof is my next best pick - cheaper and stays put, but not quite as amazing as the Benefit one.