Less than ideal-
If you have a Schwintek slide (horizontal rails on the outside of the slide) they are supposed to be opened and closed completely. If you partially open them, the motors can become out of sync with each other and cause problems. I am experiencing a problem with mine now that might be a result of doing just that.
Also, slides WILL leak water unless open or closed completely.
I've never used slide toppers, so perhaps I don't know what I'm missing, but we're in our 10th year of using slides and I haven't wished I had them. I didn't realize that they would be susceptible to high winds like an awning is.