I don’t use the Rotate Widget but it is true that I find it rather uncomfortable. Let try to show some of the reasons.
Inherent position
I not sure but I assume the only place the Angle Widget is ‘allowed’ is on the status bar, on the bottom of the screen. Maybe I’m unhelpfully lazy, however the travel distance from drawing to the bottom of the screen is kinda of tiresome. 
On my case it doesn’t help that my tablet has a drawing area in the 16:10 ratio, so I made the bottom ~1cm of it a ‘dead zone’. Which makes awkward reaching the bottom of the screen when it doesn’t match the bottom of the tablet. I usually got lost in the ‘dead zone’ strip of the tablet.
Size and “Clickability”
On my monitor that has, what I assume, an average size and resolution of 23" and 1920x1080, the small circle is a bother to click. Especially if you try to ‘fast click’ on it using your peripheral vision. Maybe is just a matter of usage and soon the placement become muscle memory, it just that the con is too much for sticking with it. (The con being having to avert looking at the canvas to the circle)
Another thing: When used I would click as soon the cursor were above the circle. With the widget always being on the bottom right corner, it led me to click when the cursor were on the left upper portion of the circle. So my canvas always snapped to ~130º. (Again this is me being a dork)
Pivot and “Area of influence”
This one is the most empirically troublesome and unavoidable. Like, I don’t think there a way to keep a circular widget and solve this.
Because the pivot is based on the widget itself, and thus at the bottom of the screen, you can’t freely rotate the canvas in angles between 180º and 0º (the lower half of the circle).
Here a visualization of what I mean:
So… yeah. Never was a fan of the Angle Widget, but with so many other ways to rotate the canvas it never bother me.
Although a slider is the more straight forward approach, like it or not. Maybe a good implementation would be a slider that you can:
- Choose between different (predefined?) sizes. Size being only length or length + height of the slider
- Choose placement on the status bar: Left, Middle or Right
- Have it behave similar to the Pop-up Palette rotate wheel.
- While the cursor is inside the slider it snap to predetermined angles
- Outside it rotate freely and further the cursor from the slider finer the control
Lastly, @Hoppa_Joel did you find bothersome using the Pop-up Palette? I find the more comfortable option, as it appears basically on the cursor location, it needs only one button to access and has snap, rough and fine rotation without the use on any modifier key.
Chees.