Okay, I tried it. It's okay, but nothing to go crazy about. Very short. And the controls are incredibly counter-intuitive, especially after playing Heavy Mach for a few days.
I played through the whole thing in about 15 minutes (I guess. Could have been more or less; I wasn't timing it). It's all rehash material (no surprise) except you are in a Slug or plane all the time. There is no reason to choose one character over another; there aren't any differences in gameplay or ending. I won't complain, though, because I love the MS2 character select screen.
The controls take a little while to get used to, and even after you do, they don't work all that hot. Even in a best case scenario, aiming takes WAAAAAAAAY too long for this type of game, as you slowly rotate the cannon all the way around to each target. The Heavy Mach touch aiming works much better and faster. And having to finger-swipe to jump makes it really awkward to try and avoid stuff by tilting to move at the same time. Seriously, try tilting and swiping simultaneously. Not impossible, but harder than it needs to be. Again, Heavy Mach does it much better. More intuitive and much easier to maneuver in tight spots while jumping.
Which is not to say Heavy Mach is the greatest game ever, either. It never does work up a real challenge, imo. And it lacks the personality of Metal Slug's animation and music. But its control scheme works a billion times better. And it's also a lot longer than Metal Slug Touch. Easily more than double the length.
Is Metal Slug Touch worth $5? Eh, probably not. But comics cost about that much now, and I've bought plenty of comics I've gotten less enjoyment out of than this, so I'm not outraged or anything. It's (a little piece of) Metal Slug on my phone, and that's kind of cool.