Romcenter

I'm trying to fix up my Sega Genesis/Megadrive files and want to keep the better versions and get rid of the bad ones. Roms with [!] are verified good yet [c] means the faulty checksum routine. What exactly does that mean? Roms like those show up as incorrect checksums on emulators, for instance. Are versions like these good anyways and the better ones to keep than ones without [!]?