I saw many recommendations for Sky Garden when doing research for my trip to London, and I can say…read morethat it's well deserved. You really can't beat a free observation deck.
Though Sky Garden is free, you still need to book a timed reservation in advance...as in, right when they release the dates 3 weeks in advance. They will run out pretty quickly even if you wait a few days, especially if you're going during busy season.
I saw that the reservation is good for one hour, but honestly, they didn't check when I went so they might not be strict on that depending on the crowd level. I went in about 20 minutes early because I wasn't doing anything. You'll check in and go through security. Then you'll get in an elevator to the top. My reservation was at 10:00am, so I got through the initial line and security quickly, but I imagine it would be a long wait later in the day.
The view out is great, and there's a balcony so you can take pictures outside. I think the interior architecture was the highlight though along with the greenery. There's so much natural light that gets in the room. It would have been amazing to go during sunset, but that didn't fit in with my plans. Luckily, the view in the morning was good, and the sunny weather was on my side. They have a cafe and a restaurant there. Though overpriced, I still got a flat white to sip on while I took in the views.
Overall, I spent about an hour at a slow pace. You definitely don't need that much time, so if you have the chance to visit, it's definitely worth it. Only issue was the line to go back down took longer than expected since people don't maximize elevator space.