I bought a 2015 BMW M235i last April. White 6-speed with red leather interior. Had previously owned a 2005 Mustang GT that was bought new. Prior to that were 2 other Mustang GTs, and 3 Camaro Z28s, as well as a 1971 442 W-30 and a 1965 Corvette 396 roadster.
I cross shopped the BMW against the 2015 Mustang GT, because the 6th gen Camaro wasn't out yet. Last month I sat in a 6-speed Hyper Blue 2SS with the Ceramic White leather accent interior, Navigation, MRC, and NPP at a local dealership. MSRP was $45K.
Value-wise, there's no comparison. The 2SS has so many standard features it's crazy - heated/cooled seats, heated wheel, HUD, rear camera, Wi-Fi hotspot, wireless mobile charging, etc. On the BMW, I had to pay $2150 for Navigation, $950 for the rear camera, $1450 for leather, $2000 for optional exhaust with rear diffuser, and I still didn't get heated seats or wheel, HUD, Apple Car Play, etc. And the GM interior quality is no joke - I was impressed.
That said, the M235i is a very high quality car - the best I have ever owned. There's attention to detail everywhere, and the QC is evident.
Subjectively, the M235i is much subtler than either Camaro or Mustang. To everyone but enthusiasts, it looks like any other BMW, so in that sense it's a sleeper. Do you want to stand out in the crowd, or blend in? There are pros and cons to each.
The real difference for me is the size. The M235i is 174 inches long, which is about 14 inches shorter than a Mustang or Camaro, and it's narrower by about 5 inches. The small size makes it nimble and super easy to drive. And fun. It just feels like you're going faster in a car that's smaller.
Anyway, would I trade for a 2SS now? It sure is tempting, I will say that.

Mine is not even a year old though, and there are still 3 years worth of free maintenance and warranty remaining, plus probably a big depreciation hit if I did trade or sell it. And, I love the car. It's modern, quick, handles like a sports car, and rides and feels like a luxury car. So now that I have it, I wouldn't trade it.
But if I was cross-shopping the M235i, Mustang GT, and 2SS right now, there's a good chance I would go with the 2SS.