This is a great plugin. Only one issue: The caching system doesn't quite work right.
Basically, a new QR code image is created with every request and stored locally, every time the page is loaded. This means I had nearly 54,000 QR code images stored in the cache for a blog that only has about 200 total posts.
It may be that the problem lies with WP-SuperCache, which I (and pretty much everyone else) use to limit server load.
Whatever the case, and other than that problem, it works great. Fix the cache issues and it's one of the best plugins out there.