Bad. Really, really bad. Like, do yourself a favor and hunt down the GBA versions of the games because they're worth the few extra dollars bad.
EDIT: Actually, let me elaborate a bit.
FF1: The best version of the game is the PSP version, since it has the most content in addition to the updated mechanics of the GBA version. The PS1 version has updated graphics, but retains the very dated mechanics of the NES version.
FF2: I'd actually suggest skipping this game, since it just flat out hasn't aged well. The PSP version is again, technically the best version for reasons listed above but if you just want to faceroll the game the PS1 version has some crazy power leveling glitches you can exploit.
FF4: The DS version is the best version, since you get updated graphics, mechanics and voice acting. Although it does retain the grindiness of the SNES version, as does the PS1 port. If you want the game with minimal grind I suggest checking out the GBA version.
FF5: The GBA version wins hands-down. The PS1 port suffers from a translation that swings between bland and horribly hilarious. Not to mention the GBA version comes with updated graphics, new content and a hilarious script.
FF6: Both the GBA version and the PS1 version are pretty solid, although the GBA version is better with its additional content. Plus the PS1 version has some pretty bad load times if I recall. However, the GBA version made a number of controversial changes to the script, so if you're attached to the original keep that in mind.