I bought a few off of Amazon to test out, since they had free returns. The non-MagSafe case I've been using is from
ANLCQC, since it best fits my preference for a thin, clear case with minimal screen lip.
Proker MagSafe Case
Only minimally thicker than the ANLCQC, but a much taller screen lip. Didn't really care for the design, though.
RESTONE Clear Magnetic Case
Adds way too much thickness to the phone. And the sides are a bit rounded, which spoils the flat edges.
MagEasy Magnetic Case (from SwitchEasy)
Adds a bit of thickness, but feels good in the hand. The MagSafe ring is only a circle, lacking the "teardrop" magnet which is meant to orient it on MagSafe mounts. Though for the other cases and car mounts I tested, that orientation magnet doesn't seem strong enough to do much.
All 3 had a good, strong hold on a MagSafe car mount. But none fit my preference for a minimalist case, adding too much heft, so they'll all go back. I'll keep waiting for one of the slim case manufacturers to add built-in MagSafe.