First of all, my setup:
Windows 7 Home Premium, SP1; fully updated (with last update being KB2679255)
GIMP 2.8.0-RC1; installed using the "gimp-2.8.0-rc1-setup.exe"
Graphics Tablet: Hanvon Art Master AM 0806

None of the following occured when I was using GIMP 2.6.x (I'm deeply sorry for not remembering the whole version)


When using my graphics tablet, the position of the cursor on screen is displayed correctly, also I'm able to use any of the menus. The small dotted indication of the brush-shape however does not follow the cursor, but stays where I last moved my normal mouse or whereever I reentered the canvas after going to the menus with the cursor; if I start using the mouse again, the brush-shape won't follow the cursor until I leave and reenter the canvas with the cursor.
I am not able to draw anything using the tablet.
Using the on-pen button, which is configured to act as right-click, on the canvas results in (most of the time) nothing, the context menu opening like it's supposed to or a small canvas-pan (the kind that would normally happen when pressing <space> while moving the mouse).

According to the "Edit > Preferences > Input Devices > Configure Extended Input Devices"-Dialog, my tablet is being recognized as a Wacom Tablet, split into three devices, which were all deactivated in the beginning. Since the results differ, I'll describe what the results are, when activating them one at a time:
1.  WACOM Tablet Eraser Stylus: Activating this differs in no way from having all turned off.
2.a WACOM Tablet Pressure Stylus (activated in screen mode): In this mode I am actually capable of drawing, however, the axes are assigned wrong. Applying higher pressure results in the brush-shape moving down, starting from the upper border of the screen. Moving the pen downwards on the y-axis results in the brush shape being moved to the right, starting from the left border of the screen. Moving the pen in any direction along the x-axis finally applies the effect of whatever brush or tool I am using right where the brush-shape is being displayed; when using brush dynamics, faster x-movement results in higher opacity. I would therefor assume that the x-velocity is being mapped onto the pressure-value with an eventual minus being ignored.
2.b WACOM Tablet Pressure Stylus (activated in window mode): As in case 2.a, the pressure is being mapped onto the y-axis, the upmost position now being the lower edge of the ruler separating canvas and menu bar. However, moving the stylus along the x- and y-axis does move the brush-shape or draw on the canvas. Instead, moving the stylus along those is now required for the brush shape to update it's position. Also, moving them may cause context menus to appear or canvas-pans to happen, as described earlier for the on-pen button.
3. WACOM Tablet Puck: Entering the canvas with cursor now automatically selects the "Move"-Tool. Moving the stylus around, no matter if touching or hovering over the tablet, once again causes random context menus and canvas-pans, but on a much higher rate than before. When none of both happens, the move-cursor is accompanied by a little prohibited-sign.

Now this was a long bug report (however, you did ask for the most detailed observation possible and here it is).
If you have any further questions, feel free to contact me.
I know what a pain debugging can be and really appreciate your hard work. Thanks a lot!