I think one of the best things about Perks is the script, or better the way the story was constructed and presented to the audience. Although it follows a typical growing up/teenager path, it's filled with small details, some horrifying, some original, some funny, that make the whole experience more interesting to watch. For example, it's not just about an awkward guy trying to make friends, it's about a boy who has suffered many loses in his life and who is now trying to recover and adapt to this new environment. The relationships are realistically built, for most parts, and I liked their interaction- although some scenes were rather common, others thrived in emotion and creativity. I especially loved the ending and how it was built to reveal the thing that has been haunting him for all these years- it made me cry like a child and probably gave me the lasting impression that Perks of being a wallflower is a great film.
Filmmaking-wise, there were a few things that stood out for me: the score and soundtrack, featuring The Smiths and New Order, the cinematography and directing in some scenes and the editing. All helped make Perks a much better film and integrate it in that time period, although I admit I realized about half way through that the action was happening in the early 90s, and not in present times.
As for the casting, there were very few mistakes made. I loved Logan Lerman's performance and, of course, Ezra Miller was his usual talented self, but I do think Emma Watson was slightly unsuitable for the part. I bet Sam was probably much more of a bad girl on paper than she was on screen, but then again, Watson's sweet demeanor doesn't help, either. Surprisingly good performances by Mae Whitman, Nina Dobrev and Dylan McDermott.
I may be biased, but I am willing to say with conviction that Perks of being a wallflower is a film you must see and enjoy. Don't take it too seriously, just remember your own adolescence and admire the character development and storyline. Try it!