I think the most impressive part about RÉSO is its sheer magnitude. It's pretty much an underground mall that spans over 20 miles. If you're a shopaholic, this is the place for you. Otherwise, it's just a really, really, REALLY big mall.
There are lots of great things to do and see in Montreal, but if you're looking for a way to kill 30 minutes or so - just enter RÉSO through one of its many entrances throughout the city and try not to get too lost.