Hi BMatthew,
Likewise I had concerns but was very pleasantly surprised with how compliant my Coupe is. The seats are comfy and the ride is A-OK. DON'T ignore speedbumps or curbs!

You'll be sorry!
Back injuries are a very personal thing. Every one is different. Keep in mind the Smart is a very small car, and if you are used to a larger, heavier, longer wheelbase car (ALL of them are), the ride will be different from what you are used to. Not necessarily worse, just different.
If possible, visit your local dealer and take an extended (1-hour?) test drive.
Good luck!
Terry