I use the Sandy's at night for my daughter, and have done since she was about 1 month old.
They are totally leakproof, and brilliant for holding lots of wee for long periods of time (she sleeps for 13 hours a night usually). They do exactly what they say on the tin. They will hold everything you throw at them and are extremely robust.
But I will say, that you have to be very careful when putting them on baby. If you leave any towelling nappy poking out from under the wrap, you will get leaks. This is easy on a placid little newborn, but try it with a wriggly 6 month old, who's tired and it's a bit more tricky.
They wash easily and tumble dry well, although I have found they do take longer to dry than the Bumgenius v3 I use during the day.
The downsides I have found so far are:
1) The wraps come in different sizes, and they are expensive (about 9 or 10 quid per wrap, and I have 7 nappies) so as the baby grows you keep having to buy the next size up which gets pricey as I started with Xsmall and I'm now on Medium. That's a lot of dosh.
2) The design is a little old fashioned. They aren't glamourous, or have particularly nice designs and aren't very modern.
3)The velcro wraps can rub- where the velcro joins across the tummy, my daughter sometimes has red marks. I'm more keen on the popper versions of the wrap.
4) They are very very bulky. Once you have the liner, the absorbant pad, the Sandy's nappy, and then the wrap, for any child under the age of about 6 months they are massive. I am not too worried, as I've only been using them at night, but I would imagine if a child was to use these during the day and was trying to learn to roll over or crawl, they would struggle.
5) This is due to them being bulky, but I have found that I've needed to buy vests that are slightly too big to fit them round the nappy. I'd imagine under daytime clothes they'd look a bit strange too.
I am planning to use the Motherease Onesize once I've finished using the Sandy's, and again, only at night.
In conclusion, I would say they are a good nappy, have a good reputation for being leak-proof, and that's entirely justified in my experience. But if you want something lightweight that fits like a disposable, that's not too expensive and relatively modern-looking, this nappy is not for you.