The digital stuff always has more of a delay than analog stuff. You will notice a delay between when you change speed and rpm, and when the display actually reacts, especially speed. Overall accuracy and accuracy at steady speeds will be good though as long as it's set up correctly, you need to program the tire roll-out for it to meter speed correctly.
Honestly, i don't think you'll use it as much as you think you might. Like octane said, it's pretty much a novelty thing on a quad. I had a trailtech endurance on my warrior and got rid of it because it was just cluttering up space on my handlebars and the buttons stopped working right from getting packed with mud. When you're riding you just forget that you have one at all, and just focus on riding. I never looked at it at all except once in a while on a long stretch. Same thing with my dual sport bike. I use the speedo and tach on the road, like with a car, but when i'm riding off road the entire instrument cluster might as well be gone. There's too much going on when you're riding off road to pay attention to stuff like that. I'd save your money for something else unless you really think it would benefit you.