Needed this as I commute with my guitar, a standard sized acoustic steel string guitar, mainly by cycle, and rains can be quite annoying with a guitar bag with a cloth-based outer layer.
Pros:
It does protect my guitar bag from the rain.
Cons:
Size. I was told this would be slightly flexible to adapt to standard guitar bag sizes, but I have to put in a significant amount of effort to put this cover on my guitar bag, as it's either slightly smaller or just the same size as my guitar bag. Taking it off is also a bit of a task due to the tight fit.