I picked up one of the MD2 UV units. It is very fun to play with as anything white will light up in the dark. In my completely dark basement I was able to navigate adequately due to the fluorescence of all the white paper, cloth and random items.
This light runs much better and "brighter" on 2 CR123s than an 18650. There is a visible difference in the distance and brightness of fluorescing items in the dark. I found a tennis ball in the yard one night doing beam shots that I didn't realize was there until I just scanned with it.
Do not get this if what you will see will bother you. I don't have a lot of experience with other UV lights, but this one is fairly powerful compared to the others I have used. My kids bathroom looked like a nuclear waste site under UV light. It might make you want to clean things you didn't know were dirty.
You will also see imperfections in paint, particularly around trim pieces. Lint/ dust on clothes is easily visible.
This is an excellent light to examine how well someone has cleaned your house. Dust lights up blue, other things are readily visible as well.
Outside some insects, insect(?) eggs, rocks will fluoresce, which is interesting. You will need to be fairly close; within 5-10 yards to see small things like this.
It may or may not be a good thing to bring traveling to a hotel.
I don't have a professional use for it and it is more of a novelty for me, but I will be using it next time I purchase a house, and it will be used to critique house cleaners moving forward.