I recently picked up a couple of 42's, with the same thoughts you seem to have about them. I was hoping Glock was going to do it in 9mm, but I still couldnt resist the 42, even though it is .380.
Now that I have them, Im not seeing how Glock will accomplish it in 9mm, if they even bother trying. I noticed pretty quick, that the 42's are really not very much smaller than my 26's. Side by side on the table, until you look at them from the rear, or head on, you really dont see any difference in size, its just the 42 is a "tad" thinner. I was thinking of replacing the 26 in my Smart Carry with a 42, but really dont see the point now. The 26 is a 9mm, holds 4 rounds more, and its back up mag is one of my 17's mags, which the 26 can use. Cant get any of that from the 42.
These days, I normally carry ball in my .380's. I used to use the hotter, big brand name HPs, until I started seeing the performance/lack of penetration failures in the gel tests. The ball rounds in the same tests always penetrated to or beyond the normally accepted penetration depths. The 42's are accurate and placing your shots really isnt an issue, even at distances most probably would not consider shooting a .380 at.
One thing about the 42 that is different than most of the .380's is, they are soft shooting, have good sights, and are more easily shot well. My SIG P238 was similar, "when it worked", which was rare, but even then, I didnt like it near as much as these 42's. Even my old standby P230, isnt near as much fun to shoot, and since its blowback, its more noticeably snappier to shoot, something I really dont remember it being until now. It has a more hand filling grip, which helps negate the recoil impulse better than most of the other .380's Ive owned. Thats not necessary with the 42's, as they fire from a locked breech, like their bigger brothers.
The one thing with the 42 that I do notice pretty quick when shooting it is, it runs out of ammo a lot quicker. Feels like Im practicing with my 642s.
I quit using my 642s when I got my first 26 and quickly realized the advantages. I easily see the 42's replacing the snubby's at the third line spot now, but it doesnt look like the 26's are going anywhere for daily back up any time soon.