I have a Peterson StroboStomp, and I absolutely love it. Not only does it tune incredibly accurately (makes setups/intonation that much sweeter!), it actually trained my ear, pitch wise. I can hear pitch more accurately, and more importantly, tell when things are out of tune a hell of a lot easier than I could before I got it!
http://www.gvst.co.uk/gtune.htm
At the end of the day, is the tuner going to actually make the guitarists any better? The one above is the best I've used and the best I care to use (and it's free!).