I’ve had a few Jetech ones in the past (available on Amazon for generally not much money). The silicone Sport bands are _ok_ but the metal studs rust a little and the silicone isn’t as flexible or comfortable as the official ones.
The leather loop straps are nice and very comfortable but longevity isn’t great. The ‘leather’ comes off the edge of the strap and it just falls apart after a while.
I also have the Milanese loop in black. I don’t know how this one compares to the official Apple Milanese loop. It looks great but I save it for smart occasions because honestly it’s just not very comfortable. Also, the weight of the watch is greater than the pull of the magnetic clasp, so the strap gets loose around the wrist pretty easily.
In conclusion, and I haven’t really thought about this in such definite terms before, I can’t recommend them. I have a couple of Apple Sport bands and an Apple nylon strap and they’re both great and look fantastic. I wear them all the time. The nylon bands look really smart.
