First, I think it's a great concept. The cost is $5 a day to participate, paid by credit card at one of hundreds of rental stations around the city. Or, you can sign up for a month or an entire season. (Stations are deployed from May to November.) After that, you simply insert that credit card and check out a bike. The first half hour of each rental is free. When you're done, you return the bike to any other station. If a station is full, you register and are then given a free fifteen minutes to return it to another nearby station.
However, the implementation has some issues, mostly related to lousy maintenance and unreliability. It's not the bikes, though. The bikes are mostly fine. It's the stations.
The failures I've encountered have been of two general types: being unable to get a bike, and being unable to return one. In my experience, each happens about a third of the time, which makes practical use of the system rather difficult.
I've run into stations which are simply dead; they're full of bikes, but the sign-up/check-out kiosk is dead, so the station can neither check out nor receive bikes. Then, there are stations which have apparently working kiosks, but all the bikes at that stand fail to respond to the unlock code the kiosk generates, so you can't check out a bike. This gets frustrating, as you end up passing other stations which are entirely empty because other nearby stations are broken and everyone who wanted a bike had to go to the station which was working, which now has no bikes.
Once you have a bike, it's fun. As I said, the bikes are mostly in good working order. I ran into one which had the end of the left handbrake lever broken off, but you could still apply the brake with full stopping power. The bikes come with integrated LED head and tail lamps, so you're reasonably safe even at night. And you can really move fast on a bike through Montreal, particularly if you're going down hill. :)
However, once you want to turn a bike in, you run into similar problems to those you had when you were trying to check one out. I have been in the position of wanting to return a bike at my destination, and had to cycle from one station to the next to find one which would accept my bike, riding farther and farther away with each station. Not because it was full, but because the electronic dock simply wouldn't accept the bike and lock. Very annoying.
So overall, I think the idea is great, but they either need to have more robust stations, or they need to spend more money on repair. There's a phone number to report problems to, but as I was roaming internationally, I wasn't eager to report all the stations. I'd have been on the phone a lot.
One of these years I'll try out the system in Paris, but it won't be until one of my banks actually starts issuing EMV compliant credit cards, which as far as I know none of them do. Bixi will accept an old-fashioned American magstripe credit card.