Your archer seems quite bent on placing himself in the open. He'd have to be in the open, in order to shoot at targets, with the occasional cover/concealment.
This makes one nice target for the sorcy/wiz who likes to use his area of effect and targeted spells.
I'd make it where their reflex save is lessened by say 1 or 2, when upside down, and possibly 2 or 3 when on a side wall.
The reflex save resembles a tumbling/dodging/lying flat effect where the avoiding PC has to roll, with the occasional feet over head result.
Now picture a PC on a side wall, attempting to dodge a lightning bolt. Remember, he's essentially fastened to the surface with those slippers.
He can move up, down, or sideways, with a movement of 20 per round, so how far can he go in 6 seconds? 1/10th of his total move. TWO feet.
The guy on the ceiling has just as much to worry about, except he can flatten himself against the surface. Still not too good.
Perhaps, you can houserule that they may have their normal save IF they relinquish their 'grip' with the slippers? At 10 -20 feet high, this might be a viable option. From the ceiling of a 50' high cavern, well, maybe not a good option.
I'm just winging it here, but, I will think on this now, as I have a rogue who just got those same slippers.