I've been using BulletProof Security on a few Werdpress sites for a while now and haven't had ANY issues with it, once its been setup. I found getting it properly setup and working with some other WordPress plugins was a bit cumbersome. However, once I got all that worked out, it's been very unobtrusive. One addition I would like to see is some way of logging security violation attempts or attacks. This way, users of this plugin would have some kind of sense it's actually doing what we want it to do. :)
Other than that, I think it's a "must have" plugin for EVERY WordPress installation!