We have an enormous set of wooden blocks that were slowly collected over the years. Some were bought new, some at garage sales, and some off of CL. For one of my kids, now a teen, they have been an incredible tool toward a real interest in architecture and building, as well as a fabulous hands on experience in spatial awareness, geometry, etc. We continue to have blocks being used to create structures, now translated to paper for more "design work". I have never regretted the cost, or the space they take up! I think it's one of those things-either your kid likes them or not. Mine, one more than the other, was just drawn to them.
We also have a seriously enormous collection of Legos here, again some bought new, lots bought off of CL, often paid for by the pound. My son loves to build the sets, but ultimately, everything gets mixed together. We have tall stacking bins of 5 drawers to store them. I would say they see almost daily use. I think they appeal differently than the unit blocks, but are another great toy/tool.
For visual/spatial kids, you really can't go wrong with either.
Also, ETA that these are the two "toys" (although I hesitate to call them that) that have survived the later childhood-into-teen years. I think they are a wise investment. Duplos, etc. have their place, but I wouldn't spend a lot of $$ on them.