This is definitely the way to go for travelling around Montreal in the warmer months. It's more convenient than the Metro and cheaper than a cab. The stations are conveniently located and $12 buys you unlimited 30min trips for 72hrs. Plus you'll get some exercise and you get to do a lot of sightseeing while biking and be able to pull over at any time.
I found the majority of the bikes I rode were in good shape, the seat could be adjusted easily, the brakes worked perfectly and the gears changed with relative ease(though there's a delay switching when biking up a steep incline).
Only two small issues we had were finding stations full when returning the bikes, then needing to bike farther to another one(apparently if you stick your credit card in the machine, it will buy you an extra 15mins to find a station with space available). The other issue was that the BIXI website is advertising 45mins in MTL as the limit per trip but they have yet to update the stations so we were both charged an extra $1.50 for a 35min journey.
It's still a bargain if you can keep your trips under an hour.