Granted, it feels a little odd when you intially enter a forward slip. It's about the same sensation as kicking out the crab angle on short final in a X-wind landing. There is a high rate of descent associated with a forward slip but that's the point of doing the manuever. You can look in just about any basic pilot training manual and see that its a taught manuever.
Anyways, I've flown some airplanes that strictly prohibit the forward slip manuever in certain flap configurations. Boeing makes no mention of the maneuver at all. I was hoping this site might have an aeroengineer or other specialty that would have more insight into this.