I had an add on stitch regulator on my Bailey mid arm, and it was hit and miss. The biggest problem was the machine itself did not have a high end stitch speed, so even with the regulator, it was really easy to move the machine faster than it could stitch so you'd still get uneven stitches.
My HQ came with one, and I've never had any problems with it. Every once in a while I do turn it off for microstitching. I love that I can set it in precision mode (starts and stops exactly how I move the machine) or the other setting where it will continue to stitch at a very slow speed (you can adjust it) when you stop moving. This allows for me to take an extra stitch at corners when changing directions, and it also stops the jerky start/stops.
I've played a bit on sit down machines with them, and didn't like them near as much. Some seemed to be not near as responsive so you'd always get a bit of larger stitches when starting to move.