Rainbow Cinema takes the stress out of seeing a movie. You don't have to fight through throngs of people to buy tickets and the theatres themselves are usually less than half full. It's also nice to get away from the bastions of consumerism that are AMC and Scotiabank. When you go there you have this kind of "we're all in this together" feeling.
You are also allowed to bring in outside food and drink and don't have to feel ashamed if you do. Every time you go there will be people with Tim Horton's coffees and full bags of chips from Metro next door.
As others have mentioned, this is not where you want to see the newest special effects centric movie, as the set up is a bit antiquated. But for dramas and comedies you can't beat it.
The prices are also very reasonable. I think the regular price is $9 including tax.