When I first saw the circles, I thought of Superman's shield, but as soon as I had a few letters at the end of 20A: 23-Across singer (MARILYNMONROE), I filled in the name and all the circles without hesitation. Well, at first I had entered it with no apostrophe and an exclamation point at the end, but when that didn't hold, I changed it to the apostrophe - which, by the way, isn't needed, I don't think, for the ABC'S answer. And really, HE'S (58D: "____ a Rebel") isn't the strongest fill either.
Frannie thought it inelegant to have the theme start on the second square of the clue, and I've got to agree. But all was not terrible. SWM (6A: Guy seeking love letters?), for example, was clever, and TARTUP (48A: Decorate flashily, informally) was fun. SLATHER (55A: Lay on thick) is a good word, and the clue for ARIA (24D: Something for trill-seekers?) was awfully cute.
There was, however, a little too much ARR, EXE, URI, AHN(?), and OUS for it to be thoroughly enjoyable. (IOU, on the other hand, was clued beautifully, so I'll let that go.) If Frannie hadn't pulled SNERD (4D: Dummy of old radio) out of her hat, we might never have gotten that NW corner finished. Had "Emeer" (variant, I know...) for OMANI at first, and couldn't think of MOC until it was practically there. Never heard of CAPEK, but Frannie also remembered KIKIDEE once that seemed the most likely. Still, the program we use (iPad app) wouldn't accept "A" for the apostrophe, so we were never able to submit it, and that leaves a bad taste.
p.s. Odd symmetry in this one. The 180º, or whatever it's called, looks strange.
p.p.s. It seems to happen somewhat frequently that a constructor has the actual and the syndicated puzzle on the same day. That is true today of David Steinberg.