Music is for everybody, good musical tastes are not. Tender Club does not guarantee perfect music all the time, but you can rely on it if you are looking for genuine rock played live by local bands.
When no group is playing, they put on a mixed modern-classic rock DJ-set and the entrance is free of charge (otherwise it depends on the band playing). Every year, in Autumn, a local radio (ControRadio) organises a Rock Contest where rock bands from all around Italy “duel” in order to get the first prize. The contest lasts about a month, but the dates are spotted here and there, mostly on weekend days or on Thursdays and most of them take place at Tender.
I like this club because usually it is not so crowded (unlike most of the clubs in Florence) and I can actually enjoy the atmosphere there. People you meet there are usually rock-oriented and easy-going. Drinks are no more expensive than at a normal pub in town.