I wouldn't put these on my sporty bike, but I have them on my city bike, and they're great. I would have bought a set years ago if I'd known how useful they are.
Others have mentioned their drawbacks and I agree, they could be better, but the problems are easy enough to work around: a bungee stops any rattle when they're closed, a little clip through the tab on the bottom piece keeps it from opening up on bumps when the basket's full, or I just tie the shopping bag handles through the rack (or together across the rack if I have one in each basket) for security. A bit of waxy chain lube on the hinges and the catch keeps the moving parts moving and fends off rust.
It sounds like a bad review but the practicality far outweighs the problems. I don't have to remove them every time I stop like I do with panniers, they're always there when I need them for unplanned shopping stops, and they carry 25 pounds of groceries a lot more comfortably than a messenger bag. 3 1/2 stars, rounded up to 4.
Pros: Durable, Easy to Use, Versatile
Cons: Could be better made, Heavy-ish