I have the GHL doser 2 and absolutely love it. My only issue is the requirement of a pc to set it up. I'm a Mac guy, but fortunately I bought my son a new pc laptop for school work or I would've had to install parallels or the like to run Windows (which I hate with a passion) on my Mac. Once it's set up, you can control it through the cloud or the app (which only works when you're on your own network), so you can ditch the pc.
It's extremely precise and super easy to adjust as your dosing needs change. I feel this doser is second to none and you won't regret spending the money. I bought mine during a labor day sale and saved $50 or $60.
Just my two pennies.