We visited for Up The O2 which is the climb over t...

We visited for Up The O2 which is the climb over the canopy. The best part was the descent back down which was actually quite good fun on the trampoline-like material (though you're definitely not supposed to bounce). As you'd expect there was a thorough safety talk before hand, lots of gear and instructions on how to use it. On the climb and descent you're connected via a harness that you have to fiddle with at regular intervals in order to pass posts that hold up the rail you're linked to. This meant that the majority of the time is spent looking down messing with the kit.
Fortunately you are able to removed the clip from the rail when you reach the viewing platform at the top but to be honest, the view from the roof was a bit underwhelming. Of course, most is obstructed by the large yellow structures and cables that protrude from the roof and recent developments mean that a lot of the view of canary wharf is blocked.
There were still some points of interest visible though.
If you're offered to do Up The O2 as a work outing, where you're not paying for the ticket then I would go for it just so you can say you've done it. It's not something that I would go out of my way to do myself though.