I've never had a problem with that Adagio Kettle, and for the price ($45.00 on Amazon right now), it's really hard to beat. The dial is accurate enough, where I usually switch between the two clearly defined Green/White and Black settings and a non-marked spot in-between the two for Oolong.
I have fairly precise temperature measuring equipment, including instant-read infrared, and the temperatures are always about where I need them to be.
The kettle has been my workhorse, getting multiple uses daily for 4 years running. Incredibly reliable and the only maintenance I've ever done is boil some white vinegar to clean out some water deposits.
Now, having said that, if I could go back in time and redo my setup with what I know now, I'd probably have gone for this electric kettle:
http://www.amazon.com/dp/B005YR0F40/?tag=neogaf0e-20 It looks a little silly, but it has some excellent features, including precise control from 140 degrees and up. It's only 80 dollars, and, considering how much use you'd get out of it, it really isn't that expensive.
So, basically what I'm saying is that the Adagio kettle is great, especially considering the price, but if you're willing to spend a little more, there are features you definitely want that the Adagio kettle is missing.