This morning, the roof terrace on top of One New Change is scheduled to open to the public. It'll be open every day, between 6am and 1am, and access is free.
We went to the launch party on Tuesday evening and, despite the plenitude of low-level celebrities (shout out to Vanessa Feltz!), the view of St. Paul's Cathedral made it a worthwhile visit. Even those not charmed by One New Change's contours might enjoy the terrace: it's one of the few places in the area where you don't have to look at the new building.
Possibly the best thing about the terrace is the journey there, via a glass-fronted elevator that impishly reveals Wren's dome as you travel upwards. Here's a quick video of the ride: