I am a great fan of invisible zips and use the plastic foot to insert them. There was only one problem and that is often after inserting the zip there was a little fold at the end of the zip where the seam and the zip come together. This problem is easily solved if one inserts the zip before sewing up the seam.
Just to clarify this a bit. If for example you are using the zip at the back of a skirt insert the zip and then sew up the back seam, it works like a charm.
The special invisible zipper foot also doubles as an ordinary zipper foot? The plastic base slides over to either to side and allows one to sew right next to a bulky seam. You don't need to adjust the needle position at all.