FNL was a lot of fun. It was my first time going and I was not too sure what to expect. The event is a combination of music, dancing, food and drinks. There are a number of food stations throughout the ROM all featuring local, food truck style offerings. We did four or five of them, from tacos and Japanese style fried chicken to ice cream sandwiches (the best!) and apple pie.
The drinks are all $6 and the beer/wine selection is not great, but good enough I guess. I would have preferred to see some local beers, but Perroni is obviously a sponsor.
The main dance floor is on the first level. The DJ was pretty good when we got there, but by the end of the night they had switched to a live rock band which was not as conducive to dancing. The stage on the second floor had a great band from Montreal. If you go make sure to check the schedule and plan which acts you want to see.
As others have said, make sure you buy tickets online first. It gets cold fast waiting in line if you don't have tickets. Also make sure to stock up on the ROM bucks so you don't have to waste time in line buying more all night.
In terms of the crowd it is a real mix of older and younger, people dressed to go clubbing and others in pretty casual clothes. My advice is to dress up more than less, but either way you will fit in.