I actually did this a few months ago!!
Well - from the inside, lol. I visited Dubai in September, and one of the things I had to do was to go up the Burj Khalifa
The elevator takes around one minute to go up all 124 floors to the observatory, and has a projection system playing on the walls, which mimics the outside as you go up which is cool. There are other floors above 124, but they're not for the general public (I think they're corporate suites, etc). On the deck you can actually walk up to 125 which we did, so you can get slightly higher and look down onto the deck. See some pics I took below.
Before I went, a buddy of mine told me to prepare to be sick as it 'wobbles' when you're at the top there - but i felt no wobble at all.
All in all, a great experience if you happen to visit Dubai.
