I have had the 18x20 frame since 2005 and a top will last 3/4 years depending on weather.  I have had to replace a few vertical legs over the years.  Basically I have been satisfied with the canopy.  I have designed special anchors to hold it down.  Under each leg I installed an auger and then pinned the legs to the auger's.  In addition, for the tie down loops I add and extra knot to the string which makes the loop fit tighter and the rain wont fill up on the sides.  I also addeda bungy cord to the four corner tie down's to give them some flexibility and they won't come loose.