Just before buying these adjusters I'd picked up a pack made by Belkin.
I found both to be adequate and can't say either has an edge.
The Heininger actually stay attached to the belt between uses, which is a plus. But unsnapping them once properly attached can be tricky. When being used, these adjusters need to be slid to the side of a belt that is pulled tight and low across the lap...which is the proper way to use a belt anyway. If they aren't secured this way, they tend to excessively slide/move along the belt while in use if you happen to move much.
The Belkin have a more in-use secure fit, but slip right off the belt when not in use do to their 'open on both sides' design. Also, if you like to wear your belt a little more loosely, these are the better option.
Overall, I don't think there is an optimal product for adjusting seat belts out there to date, so this item is no better or worse than others.
Being barely 4'6" and having to drive with the car seat far forward to safely reach the accelerator/brake pedals, my neck being painfully chafed by the shoulder strap of my seatbelt is a problem with which I've long been accustomed, so I had high hopes that this product could offer some relief. Does it? Yes and no. I expected a permanent solution--i.e., something that I would install once and then not have to fuss with every time I got in and out of the car; this is not that. … more
With the Seat Strap AdjusterTM clipped to you seatbelt, the shoulder strap will sit where you want it to. Still providing an effective working seatbelt. The Seat Strap AdjusterTM is made to work with a three point seatbelt, making the shoulder strap maintain an upright position in case of impact. This innovative product takes into consideration that not all standard seatbelts fit everybody, offering safety and comfort in one simple package.