You know, it's really amazing what about 4-5 oz of DIY refrigerant will do for your A/C system.
I got bored this afternoon and decided that was a good quick project, and boy is the air blowing a lot cooler in my car. Temp seems to be equalized between driver and passenger side now, too.
I can't explain why my car was low on freon -- I'll just blame the prior owner. I know it can't go somewhere without a leak -- as Matt L says, something greater is probably wrong. But for now, and for $5, I'm a fairly happy camper. I'll watch and see if/when the performance degradation comes, then have a leak check done or something.
Cheers, John