They have a cool free rides promotion - you get a free ride if you use a code from a friend mine is offerr1ue - feel free to use it (we'll both benefit)!
Uber is great when there's enough available cars in your area. However - that usually overlaps with high availability of cabs. I like their price estimates - it's good to know in advance! It's also easy not to worry about payment since it's charged to your credit card directly. Some say it's not as safe as cabs - but with the Uber rating system I feel very safe to ride with a driver that has good ratings. It's also good for people that leave their stuff behind in the car (easy to get in touch with the driver and retrieve the lost items) - but it never happened to me. We usually take an Uber X, but once we were moving some stuff and got an Uber XL. Whenever I take it I am happy with the service. There are a few downsides too - the Uber navigation software that the drivers use doesn't always take you from the best route. The waiting time estimate can be misleading - and it often takes more time than initially estimated. Sometimes you count on getting an Uber and there aren't cars available, or they hike the prices - to an even higher price compared to a cab. It happened to me when I was counting on an Uber to go to the airport late at night (or early in the morning) and I ended up hailing a cab after walking in the rain to a main street and waiting for a few minutes.
In a nutshell - it's my default option - but you can not rely on just Uber in Montreal.