Benjamin Atkinsonwrote a review of Kryptonite Messenger Mini U-Lock on August 1, 2019
Familiarity: I've used it several times
When it comes to picking bike locks it can be a real hassle balancing security concerns with practicality. I really like this lock because it is small enough and light enough that it isn't a burden to carry around, yet it is secure enough to safely lock up a bike under normal conditions.
I'm never away from the bike for more than a couple hours at a time and I always lock it in either a high traffic area or hidden. I suspect that cutting through this lock would likely require an angle grinder and make an awful lot of noise and take probably 10 minutes or so.
Most bike thefts are crimes of opportunity so you just need to make it difficult enough that the thief moves on. If someone is already coming at your bike with an angle grinder in hand, I'm not sure that there is realistically much you could do to stop them.
Another big benefit to this lock is that it is small enough that I can easily loop it around my head tube for transport and it doesn't interfere with my ride at all.