Picked up one of these little gems this weekend. I'd been using an audio-based tuner for years, but when I recently started teaching workshops, I quickly learned how hard it is to tune in a roomful of ukers.
There is something wonderfully satisfying about seeing the tuner screen turn green when you're in tune. I regret not having gotten one sooner. Monkey simple to use, and works for both C- and D-tuning. Highly recommended! Link