They're tougher to find but not with great demand owing to the sizes and usually ragged condition. I happen to really like them and have quite a few.
Series 1 I appreciate the clear, clean pop art-style images:
Nice thing is that if the kid messed up the border partially, as he did with the Klein, you can bob it down the next layer of frame and it still looks good:
The instructional ones can be a little more ragged:
One thing that is clear with handling lots of Wheaties boxes is that kids in the 1930s had very poor hand-eye coordination because most of the box cards I see have miserable cuts. As do most strip cards.