I went there with my Grandsons, it was lovely. It is free and you do have to book, and take ID, although the children to not have too. They also have a lovely coffee shop and a restaurant, and a very good seating area. My Gransons loved the views , and say in the little seating area's in the gardens were good too.
The building may have won the Carbuncle Cup award but the Sky Garden at the top is well worth a visit. I took DGD yesterday - the views from the observation decks are great. It's a fascinating space, huge, and free of charge but you do have to book.