Warning - tourist trap ahead!
I like 'old' but I'm not sure that I would call Old Montreal 'old.' Maybe 'older' but not 'old.' There is certainly plenty to do here (shop, eat, drink) but most of it seemed more geared towards tourists than locals. When I travel, I tend to like to go where the locals go and avoid the touristy places so I didn't spend much time here after checking it out.
Most of the restaurants here serve the same things, for the same prices, but it is nice to be able to sit out on the patios and enjoy some nice weather.
Definitely check it out, but don't plan your entire trip around it.