They are HEAVY. If you are used to lighter weight machines, be prepared for this. While it gives you stability after the initial shock, I had a bit of a weaving phase before finding my feet as it were.
Key problems. I have had a few already. The cycle hire line is pants and it's obviously not coping with demand. Don't be put off. At least twice I've called back and gone through to the centre itself instead of the overflow. If they have to reactivate your key and it doesn't work immediately, it's worth trying it again after about 10 minutes. I found walking to the next docking station filled the time and destressed me.
Redocking. Do a BoJo. When he says be firm, he's right. I've seen several people insert their key to redock their bike. You don't need to do this. Again, take both handlebars in a firm grip and roll the bike at a reasonable speed into the dock. Hold it in while waiting for the green light.
Do your research. While I've not yet been unable to get a bike at my dock of choice because of a lack of machines, it does happen. I have memorised the nearest four docking stations to the two stations I travel between so that I can just walk to the next one. Some of them are in weirdly residential places and bikes are always plentiful in these.
Persevere. The initial key problems caused me no end of grief and I got really annoyed. I refused to be cowed and decided to keep trying and it's been worth it. I travel into London and out again to get to work without having to use the tube. Lovely.