When my 3x3 times started to get below 45s I thought it was time to try a 4x4. This seemed a good price and Shengshou have a good reputation in the cubing world.
It's great! The cube itself is smooth turning out of the box. I run my 3x3 Zhanchi pretty loose, so I initially had some problems with lockups, but that's just down to my sloppy technique. It will corner cut a bit, but after concentrating on making good accurate turns it hardly ever locks.
It seems very durable - the stickers on this have outlasted a set of 3x3 stickers very comfortably, and are only just starting to chip at the edges after many months of daily play. (A set of hard tiles for these is only a few dollars from Cubesmith. They will last much, much longer again.) It has even suffered falls onto hard floors from desk/shelf height with no apparent ill effects.
If you can solve a 3x3 this is a nice step up. It takes me about 5 times as long to solve this as a 3x3, so it's a better way of killing time, and there are lots of decent video tutorials online to get you started. Or if you know an over confidant child who needs something to keep them quiet, look no further!