I've seen people here complain about this messing up their monitor and giving it a strong yellow or magenta hue.
I should point out, when I first started using this, it did the same thing (sort of, I calibrated my main monitor, then when I tried to calibrate my secondary monitor, the main one started losing it's hue while it was calibrating, and ended up with a deep yellow hue).
I reset my monitor and then went and downloaded the newest version of the software (I'm on a Windows machine, so I don't know how well this would work on a Mac). Now it works perfectly fine on both monitors.
I've been using this for about a year now, calibrating every few weeks. I have two monitors, both completely different makes, and therefore it's difficult for me to manually match colours between them. I do a lot of work in Photoshop using both screens so it's important for me to have the colours match as much as possible.
With this, for the price, it does the job pretty well. It has a function where you can set the sensor down on your desk, plugged in to your computer and it'll dynamically change the colour profile based on the ambient light in the room. It's a neat feature, but I don't much like the idea of my screen's colours changing randomly, so I don't use that feature (and I find making a colour profile with ambient light as a consideration ends up with the brightness turned up slightly, but you can choose to calibrate without using the ambient light).
Otherwise, it's fairly easy to use, the software guides you through everything and it only takes about 5-10 minutes per screen to calibrate (and it does it mostly automatically, so you can go make yourself a cup of tea or something while it works).
I'd recommend it, there's likely better, more expensive alternatives out there, but for the price, I think it's a good deal.