As some will know, I used to work in a Subway for about 2 months. It was horrific, and I've only been able to eat there since when there's been no other choice except hunger. Working there, I mean. But, depending on what you want to eat, it can be okay.
Top TOP top TOP tip: no 'dressed' chicken.
Also, the red onion was always incredibly nasty, and frequently rotten. And er, used to make sandwiches anyway.
If you are vegetarian, and it looks even *slightly* busy, insist they change their gloves before prep. They can't refuse to do this.
Try and avoid getting one during any kind of busy period - it's company policy to put on new employees during these times, oddly, so you're likely to have to wait for ages AND get a shitty sandwich by some newbie fuckwit.
Burger King > McDonalds > KFC > Subway