Exapunks was my first Zach-like and I loved it. It and most other Zachtronics games have a very well-tuned difficulty curve that pushes me out of my comfort zone just the right amount. I think getting stuck for short periods of time makes for a good puzzle game.
I finished Opus Magnum a couple weeks ago and I found it a little frustrating because of the same reasons you brought up. The game doesn't force me to be clever; I can be as simplistic and inefficient as I like. I did go out of my way to design a couple efficient designs, but it didn't feel especially rewarding.
FWIW, my favorite game from them is Last Call BBS. It has several great "mini"games that feel rewarding to just complete.
yea, I don't think Opus Magnum is my favorite either. I just think it's a good starter, to get a gentler introduction to the many-step puzzles and those optimized-build leaderboards - if those leaderboards call to you and you like fighting your way towards the left, you're going to be a Zachtronics addict in no time.
if you don't enjoy them, then you've at least had a sizable taste of the mechanics and know why it's not your thing, rather than getting hard-blocked early on because you couldn't figure out a trick :)