Pros
As far as fast food restaurants go, Subway is probably one of the better ones to work at. For one, there is no grill which requires you to be at least 16 years old to operate, so you can work at Subway once you start high school. There's also no drive-thru so the only customers you need to worry about are right in front of you. Depending on your managers and your hours, you will be required to do just about anything there is to do in the restaurant, so if you like doing different things at work or just keeping busy, there will be something for you to do. If you work at a Subway in a small town like I did, there are much fewer customers in general which means there will be times that you are by yourself, which means you are essentially your own manager while there.
Cons
As far as pay goes, if you're part time, don't expect anything above minimum wage. You have potential to get a raise around 6 months to a year, but by then minimum wage will have risen to your new salary and you won't be getting a raise again to offset that. Also, with the pro of keeping yourself busy, the con comes when there aren't enough employees around to do anything more than make sandwiches, which means you fall behind on all your other tasks that need to be done. Subway will only hire just enough employees to keep up with customer flow, and in my small town Subway, there was a time where only 5 people worked there. It can make it difficult to call in sick or ask for days off when there is nobody to take your shift.