As a Milanès, this is hands down, my favourite place in town.
It is as the same time the hall of an hostel, a pub, a concert venue... and a comfy living room.
I usually come here with my friends for a beer ( 3 euros approx for a beer, 4,5 for a spritz, slightly more for cocktails) and i get something to eat as an aperitivo for free (don't come here for fancy food, there's simple healtthy homemade food).
I like that here you are not hurried to buy, i can easily stay here for 5 hours just chatting and playing board games that are provided from the bar. In addition there's sofas and guitars you can use if you feel like bonding with new people.
Sometimes there are concerts in the downstairs part and then the air literally catches fire as you see the whole crowd singing and dancing at cm from their favourite artists, so I recommend you check on their facebook page for any upcoming event.(https://www.facebook.com/OstelloBelloMilanHostel/)
Some nights there's karaoke!
Also, the location, in via medici 4, is really near to Navigli and Colonne di San Lorenzo, that are nightlife areas in which most of young people go.
For me is a 10/10