Well-Made, Small, Finnicky, and Worth It
This is an exceptionally well-made piece of motorcycle luggage. And at this price, it should be.
It is small, but despite that, it definitely has space for typical day-ride sundries (e.g., earplugs, phone charging cable, knife, wallet, pen, notepad, portable USB power pack, Garmin Navigator VI case).
It mounts using three special Wunderlich-provided screws that come with the bag. These replace three of the BMW fasteners that come on the bike. They are somewhat like the male part of a snap button. I've found that when the bag is zipped up, it is harder to attach all three of the female fasteners on the bag to the three male fasteners around the tank. With the bag unzipped, there is more flex, allowing for easy angles by which to attach the fasteners. All the same, you can do it with the bag zipped up and full, it just takes a little patience. Note that coming from a magnetic tank bag may lend a certain impatience to getting this bag on, but as it is extremely convenient, it's worth getting used to.
Also note that when the bag is mounted (and this may be the case with any Tankbag on this bike), when you crack the handlebars all the way to the left (normally when maneuvering the bike to park it and are moving the handlebars to their extremes, left and right), the horn button will make contact with the bag and you may inadvertently activate the horn.
As an investment goes, I think this piece is worth it. Aside from the BMW-manufactured tank bag for this bike, I'm not aware of too many that don't require some sort of apparatus attached to the tank, and Wunderlich's fastening system is rather innocuous and doesn't look non-stock at first glance.