it has a nice cast but is the story good? Will the first episode get me into it right away or it takes time?

I personally loved the show, and it is definitely one of my favourite dramas of all time. It has a great characters and a great story that deals with issues in the school system. And for your second question, in my opinion it takes probably about a couple of episodes for things to get going. 
its amazing my most rewatched drama ever ..definitely worth it ..but u will like it by ep 4 as u reach ep 7 it become addiction ,at least that was for me ..
if you are into friendship / bromance you should give it a try until episode 5. that's where things get really better :)