Tagging onto this post, I went to the Belvedere with no advance reservations (because that's how I roll :)) and I had to wait 1.5 hours for my admission time. Just for planning purposes. I didn't mind because I sat outside and looked around and read my Rick Steves guide.
I also noted that this was the only place in Austria where I saw a warning about pickpockets. It was on the wall behind the ticket sellers, I'm surprised they would choose that location but there you have it.
Also to note - if you're going to see Klimt, you may as well find out which floor they are on and go there first so you don't get disappointed. This is obviously the most crowded section of the palace and it may take a while to get your fill of his art.
I enjoyed this museum even without the audio tour, and it's really simple to tram there and back. Only did the upper tour per Rick's advice, I did walk to the lower and then exit and got somewhat turned around. Make sure you're getting back on the right tram going the right direction.