My son started playing building blocks since he was 6-12 months old. There was a giant size "Mega Block". I think he did not enjoy much then.
I managed to get a set of Lego "duplo" with Bob the Builder character on it. I think he enjoyed the set, more because of the character than the building blocks. My daughter, who inherited the toys from her brother showed me exactly the same thing.
I received a pile of Lego blocks from one of my US colleague. I saw my son making some designs out of it. Frankly speaking, I did not appreciate his designs and I regretted it. I can't show you the original version because I did not take a picture at that time. Below is the latest improved version.
I called that "lu lu car", and my son was mad at me, because "lu lu", in our dialect means acting stupid but cute. Sometimes we called him "lu lu Kai" or called myself "lu lu Pa", so you can understand we did not mean to criticize at all.
I taught him the concept of symmetry and how designing something which is symmetry in one axis would look good. Look at several of his designs below. I am amazed. He has his down design of trains, binoculars, cars (improved from his "lu lu car"). I think the lego set is worth my investment because it stimulates him to think and to concentrate.
He is spoilt by his parents and grandparents with many different sets of Lego. You can see that even for a complicated one like below, he knows how to read the instructions manual and construct the blocks.
Look at my son's design, utilizing almost every piece of the Lego duplo. I can see that my son enjoy building the blocks, and my daughter enjoys playing with Bob and Wendy.
I would like to buy a cheap display cabinet for his lego designs, waiting for the sales from Ikea. And I learned my lessons using the phrase "lu lu car" on his first designs, although very unintended. :)
P/S: Just in case you need this: