They cost, but they're really unique--some refer to me as "the guy with the gauges". I wanted a tach, and was going to spend, so I figured why not. I've met only one other rider with them, and by sheer coincidence ended up sitting in the next cubicle at work.
I had to replace it once under warranty, the speedo got whacked. I hear it's not uncommon. I have the big tach version, but in retrospect maybe the large speedo would've been more useful. I'm not sure they fit newer Monsters like yours. Older ones without tachs created some demand but not anymore. Double check on that.