'Wow what a cool toy' was my thought also when finding over a hundred of these (128 actually) at a yard sale. They must be crazy to be letting them go for only five bucks. Was the second thought, but in the year and a half since it has become quite obvious why.
Actual playtime has been a max of 3 or 4 times for my five and nine year old. Although they originally were very interested and put an hour or two a couple of times and the eldest has managed some very complex structures, the fun stops when the building is done. As soon as the table gets bumped or the engineering gets a little off the walls come tumbling down, and it is impossible to play with any of the finished projects you have to leave them set.
This is my best explanation for the short term appeal, lego's, tinkertoys, lincoln logs and all their other building toys have seen way more use and especially repeated use over the years, but the finished products from those can be played with without immediately falling apart. Still have them on the shelf in case of more appeal in the future because we have the space, but if we get to many lego sets from the grandparents some future xmas that may change.
Finally a suggestion for those who have or get these, find a cookie sheet that is made of ferrous metal (one the magnets stick to) to build on, this helps projects to hold still and makes them portable when complete.