I've been here twice and had a good time both times. The first time I came on/near Halloween and it was awesome mainly because of the costumes of the guests. I think people who like museums tend to be more creative than average and so put a lot more effort into their costumes. It was fun just to walk around and gawk at all the outfits! I highly recommend coming here at the end of Oct. for that reason alone.
I just went again for the May 20 Fashion-themed night and it was fun too. The Fashion exhibits were a bit underwhelming but my favourite thing is actually just wandering through the regular exhibits. You can't take food/drinks into all of them, but unlike the AGO Friday Night event the ROM doesn't block off rooms so you can still go and see the bat cave at night. I mean, not that you would know it's night in the bat cave but it's still cool to be there after hours. It plays into my 'From the Mixed-Up Files of Mrs. Basil E. Frankweiler' fantasies (YA book where a pair of siblings run away & secretly live at the Met).