I mostly use a linux native app called lingot. It does the job but has small annoying bugs (settings file is not parsed properly, my ALSA device CARD:foo,bar=baz fails because of the colon character, I'll write a patch one of these days).
This guitar tuner works, I tuned my guitar succesfully with it but it was a bit painful. I'm on Firefox 27.0 on Linux, using Alsa, no PulseAudio. I am using a Microsoft LifeChat headphone + microphone laid on the table before me that is good enough to tune my guitar with Lingot. I would have tried Chromium for comparison but it seems I have a version that is too old.
There was a very bad latency, around 1 second from playing a note to seeing feedback on the display. The information I got back was decent and good enough to get my guitar in tune. Overall the quality of this was somewhat comparable to a cheap guitar tuner from the 1990s with an internal microphone. But I think the culprit here is the platform (ie. browser + audio frameworks) rather than the quality of the app itself.
What platform(s) have you used with tuner and how did it work out? What browser, OS and audio system did you use?
If you want me to help out testing this app, please reply and tell me what to test and I will.