Excellent but needs to be larger
I think that this is an excellent product and would give it 5 stars if the size was actually 18".  A more accurate measurement is 14 1/2 by 16 1/2.  I have noted that several other reviews have pointed out the size issue.  So unless you have a skinny tush you may find that this cushion needs to be wider.<br />Now on to how well the cushion worked.  I have herniated lumbar disc (L4 and 5) so driving long distances is not pleasant.  I just returned from a drive that was 600 miles each way (9 hours).  The Wondergel cushion made a huge difference in my comfort during the trip as well as when the trip ended.  No sore back.  I have no idea of how well the gel holds up over the long run -I've used it for about a month-but there was absolutely no compression visible after sitting for 9 hours.  A NOTE FOR THE MANUFACTURER: You have a great product but you need to make a larger version of this cushion...say 20".  I would gladly pay extra and will buy a couple of them.
Rating: 4 stars