The quality varies from mic to mic. Some are pretty bad, others are acceptable.
However, the arguments put forward tend to be as follows. First, they're simple--you just plug it in and go. Second, it's cheaper than an interface and a microphone. Unfortunately, these arguments aren't always true.
In terms of simplicity, SOME people can just "plug and go" but you need only to read this forum (or any other recording related forum) to see the number of problems people have. Computers fail to detect the USB mic or, if the computer detects it, your software decides it can't use it for some reason. Or the drivers turn out to be not compatible with your software. Or you get the input but the levels are wrong and there's no adjustment. I'd guess that of the posts regarding basic problems, USB mics account for at least 30% of the issues we get over on the Audition forum I lurk in.
Second, the price. Yeah. Initially it can look attractive but so often it turns out to be a false economy. Again, you only have to read this forum to see how many people rapidly become dis-satisfied with their shiny new USB microphone and, guess what: as soon as you want something better you have to scrap the USB mic and buy a dedicated interface anyway.
In my opinion, you're always better off buying even a basic USB interface and a conventional microphone. This combination won't be much more expensive than a GOOD USB mic but it'll set you up for future. You can add other mics and mix and match as much as you want.
USB mics are a false economy. They're not necessarily simple and certainly not cheaper once you decide you need to change something. Do yourself a favour and avoid them.
Bob