In Frankfurt subway system is pretty unusual for me: subway (U-bahn) and suburban trains (S-bahn) are connected in the single system. They even use the same tunnels in the city though different rails. You have to be extra watchful to not miss your train or direction. Detailed map and schedule will really ease your life.

However appearance is not really impressive. Especially if you compare to Moscow subway:

The only peace of art here is some graffiti left by unknown artist :)

Though who cares about the appearance? (:
Trains are pretty comfortable. Even door opening is controlled not only by train driver, but also by passengers: you can find a special button on each door which will open the door. Otherwise it will stay closed. Special indicators will not let the door close until the entrance is not clear.

This is rush hour:

Can you imagine something like this in Moscow subway in the morning? (:
Ticket price really makes me sad. Round trip to office will cost something like 6 euro. That's a lot. Though monthly pass is about 80 euro which makes the whole thing much better. Especially when you will find out that this pass is valid also for a bus. However I prefer walking. It's just about 10 minutes from S-bahn station to the office.
There is no special control gates on the subway enter. But it doesn't mean you can freely go without a ticket. There are too many guys who was hired to check your ticket. For the last 4 trips I met them 3 times. I'm lucky, yeah :) They will fine you for 40 euro if you have no valid ticket. They will not take you money, just give you a receive which you have to pay. I even saw ticket-less guy who wasn't fast enough to run away.

Ticket purchase. That's a thing! First time I spent something like 15 minutes. There is no guy who's selling the tickets. Only machine. Of course I was lucky enough to fine old machine without English translation with only button interface. My German is not that good yet. That's the machine:

However, there are some places where you can buy ticket from live person. Main train station is this place. Exactly here I purchased monthly pass.
Ah, that;s my station. So I will go. See ya (:

ps. Here's some photos: