Not long ago I picked up the Polytune Clip from TC Electronic and have since been very pleased with the way it works on acoustic guitar in particular. (Of course it also works on electric guitar and other instruments as well.) I do a lot of open and altered tunings and this tuner handles them like a champ. In addition to latching on to notes quickly and accurately in chromatic mode, the Polytune picks up the low 6th string bass notes well (I tune down as low as Bb). Tracking is great and you can see the note steadily move into tune as you turn the tuning pegs. Of course the polytune mode doesn’t really work with open / altered tunings, but the Polytune Clip works so well as a chromatic tuner that it’s worthwhile for that alone. Highly recommended.
TC Electronic: http://www.tcelectronic.com/polytune-clip/
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Video on the Gotoh 510 tuners I have on my guitar here