Mayday aren't notorious for having the highest quality sleeves, but they are cheap, an amazing fit, readily available around the net and offer a high number of sleeves for a good value (compared to FFG who give you half the cards at the same price) You will likely run into a couple sleeves that aren't cut properly or are slightly longer than others, but it is a small amount. You could fix this yourself, use it for a less important card or just toss it, since you're not wasting a whole lot by doing so. Cards can stick together after the initial slippery stage of first use, but if you have a high number of cards (CCG, LCG) and want them protected or double sleeved, then you might not have a whole lot of other choices. FFG and Dragon Shield may make better sleeves but their prices and fit may not be for everyone.
Mayday does make higher quality black-backed sleeves (that come 80-to-a-pack) and other premium selections, but those do cost more and increase the thickness of your stack, which can tip over in high number decks. Overall I think these are an excellent choice for anyone needing a large number of sleeves on the cheap and want their cards to sit snugly in their sleeves.