A few years ago I tried to hire a couple of Boris Bikes in London with my son. Despite help from an Official Assistant, we were unable to unlock two bikes at once. So Boris you owe me £15, which with late payment charges and interest comes to just over £10,000.
Citibikes in Vienna is much better! There are various apps which show you the location of all the docking stations and the number of bikes available at each one. You can do most of the preliminary registration stuff on the website. However, the final step of registering your debit card requires you to use one of the card readers at a bike station. Note for Monzo users, just tell the machine it's a credit card when registering.
Fees are very reasonable. The first hour is free and the second costs 1EU. Vienna is pancake flat with lots of cycle lanes, so even occasional cyclists should be fine cycling around easy and safe.
However, the bikes are really heavy. They would probably survive a direct hit from an anti-tank missile. There is a single front brake, but the main braking is activated by back pedaling. If you are used to aligning your pedals when you mount the bike by sticking your foot under a pedal and back pedalling to get the pedal in the correct position, you can't do this on a Citibike.
There are three gears. However, the weight of the bike means that even on slight inclines you have to push hard in the lowest gear.
If you are a regular cyclist you will find the weight and braking system very annoying. On my 6.5k ride I averaged about 16kph. On a similar route with my normal road bike this would have been about 35kph.
In conclusion good bike for short rides (10-15 minutes) at a very reasonable price.