I've had good success on really dirty wheel-wells with both Meguiar's #39 and with Extra. They are also good if there are parts that are hard to get to with a brush (I mean, I'm not such a fanatic that I jack the car and remove the wheel or anything. I bet someone here does, though). Just the spray gets a lot of dirt flowing. I imagine APC+ would work great too.
I've never tried a dressing in them. Interesting idea. Maybe I'll try the Engine-Kote on them when I get it since it would be hard to apply a vinyl protectant evenly in there. Plus, how else could anyone (that isn't a pro detailer) possibly use up a whole gallon of that stuff... :nixweiss