I don't own one, but have read several reviews that it is not accurate. One I read said that they battery was at 10 volts, but using a multimeter, the battery was actually at 14 volts.
I'm not saying don't buy it, I suggesting that if you get it to verify the reading with a volt/multimeter.
I've been using one for the past 4 years and it does what it's supposed to do, put out idiot lights to tell me that the charging system on the bike is working.
I installed it on my bike and tested it against a Fluke multimeter. It is fairly accurate and provides peace of mind as far as I'm concerned. It's only downside for me came from where I mounted it. I put it directly beneath the speedometer which places the light sensor pointing straight up. Going under some street lights causes it to brighten the display (day mode) and I used to find that distracting. I'm used to it now.
The only other downside I can see would depend on what type of battery you have. Standard lead acid and absorptive glass mat (AGM) batteries will give a good indication of charging status/battery condition. The new lithium batteries maintain voltage output much better than the other two. A lithium battery will maintain it's high output until it's very close to a discharged state. In other words, should a charging problem arise, the voltage reading will stay high even though the battery may be close to dying.
I have one mounted in a spot that the handle bars hide it from view while riding. I felt it was too bright. I can tilt my head a bit to see it if I need to,