They're brilliant. Built like a tank - scares the black cabbies away nicely. Heavy, but very low geared and comfortable. Like riding a sofa. You won't get any Strava segments though.
I use them all the time around London, but not that often it's worth my while getting an annual key. Remember, dock it within half an hour, wait 10 minutes, then off you go again. All for the £2 per day access fee. There're not many places you can't get to within the free half an hour. And you're never far from a docking station.
I don't think you need to be desperate to ride one. But equally, I wouldn't like it if they were my only option.
Go for it.