For 10€, I'd also go for first class. You can also access DB lounges if you have a first class ticket.

That being said, second class is not bad, though the IC trains are of very variable quality, while the ICE trains are pretty uniformly nice. Some IC and RE trains have air conditioning (even in second class), it depends on the train itself. RE trains are functional trains, often serving commuters. They are not always staffed with a conductor.