I really like these head covers. I've tried some others over the years, and these stand out as the best yet!
They are durable, because they are made of a high quality, thick, injection-molded plastic/rubber material. It appears that it will hold up well to years of use (but not abuse).
Also, they are well shaped for my clubs (Mizuno MP-60's). I think I got the non-oversized version, and they are just the right size. Since my clubs are soft, forged metal, I wanted to keep them from dinging each other. These fit the bill well!
They are easy to use and snap on and off quickly and easily. (They don't actually "snap", but the shape of the mold kind of makes them "snap" onto the club head.) They are easy to remove by hand, but seem like they wouldn't come off unintentionally. I've even swung a club with the head cover on it to see what would happen, and it didn't shift at all.
These are much, much better than the neoprene sleeves I've tried. Those wear out very quickly, with the stitched seams unraveling almost immediately. I also had a set of head covers that threaded together and only slid on the face of the club. Since they couldn't "hold on" to the hosel, they fell off WAY too easily and were a major pain.
These head covers do exactly what you would expect, they do it well, and I don't see any downside to them.
There is one thing to note, however: As they are a thick, non-breathable plastic/rubber material, it's not recommended to keep them on the club heads while the clubs are in storage. This will trap moisture and lead to rusting more quickly. This isn't a problem for me, though, as it takes less than 30 seconds to completely remove or put on the set, so I use them while I play and take them off when I get home. I also don't mind taking 5 or 10 minutes after a round to clean my clubs up, so this isn't a big deal for me.
Try them; you'll love them and keep your clubs like new!