The problems that I see are a follows:
1. You have to dig down several menus deep to reach it.
2. The speed control overlay remains permanently on screen and does not hide with the rest of the onscreen controls. If you close it because you want to see the full screen without the clutter, then every time you want to make a speed adjustment you have to drill back down through the menu, make the adjustment, and then close the window again.
3. Once the playback speed adjustment overlay window has been closed, there is no longer any visual feedback as to the current playback speed.
4. Because the playback speed control is a separate overlay window (rather than being attached to the rest of the onscreen controls) it jumps around all over the place when the other controls appear and disappear (causing mis-clicks if, for example, you are trying to repeatedly tap the + or - buttons to adjust speed).
What I would like to propose is a single speed control icon button (with digits showing the current playback speed) that displays as part of the regular onscreen controls. For example, it could be placed next to the HW+ button on the top control bar (although if I had my own way, I would move the lock button up next to the HW+, then move the 'fit to screen' to where the lock button was (to the left of the rewind / go back button), and then put the new playback speed button in it's place (to the right of the fast forward button). But that's just me.
Anyway, regardless of positioning, it would be incredibly useful to always be able to see the current playback speed with just with a single tap (because it would display as part of regular oncreen controls) as well as being able to easily adjust speed with from the main screen without having to drill through menus.
A slider control would allow quick adjustment of large speed changes instead of having to go tap, tap, tap, tap, tap, and it would make speed adjustments more fluid from a usage point of view. The slider could still be 'notched' to adjust in steps if the user drags slowly (so that reaching an exact playback speed value can be achieved if required).
Another idea that I've just had is perhaps there could be an option to display the speed control slider by long-pressing the play button. That seems as intuitive a place to control playback speed as anywhere else. The current playback speed could be displayed as digits either above or below the play button itself (so you'd always know the current speed).
Just some suggestions for your consideration.