Re: Anyone going to the World Tour Finals??
It's a fantastic experience - I thoroughly recommend it.
I went in 2010 and was fortunate to see a few matches - Ferrer vs Federer (round robin), Roddick vs Nadal (round robin), Federer vs Djokovic (SF - the last match Djokovic lost before going on his winning streak), Murray vs Nadal (semi final - one of the most tense 3 set matches I've seen) and Nadal vs Federer (final - Rafa had won the last 3 slams that year but Fed was firing on all cylinders, particularly his backhand).
As for tips:
1) Go early to beat the crowds and try and get a seat by one of the two smaller practice courts in the fun zones towards the back of the complex. It's amazing how close you can actually get (feel free to explore some of the other exhibitions in between matches too).
2) Hang around the main practice court before crowds gather as you won't be able to see much otherwise.
3) The doubles matches are scheduled before the singles, but try and see if you can pop into the main arena an hour or so before the doubles match starts (especially in the evening session) because the chances are that one of the top players might have booked it for practice. I did this and managed to see Roddick and Murray practice their serving. I also did this on the afternoon of the final and saw Federer warming up against Verdasco.
Tip: Move over by the corner where the players enter / exit and you may be lucky enough to get an autograph at the end of their practice session...I managed to get Fed to sign my ticket for the final.
4) A fair few former tennis legends are on site doing media commitments. If you see one just wandering about, pluck up the courage and have a quick chat! One lady was telling me how she just went up to Goran Ivanisevic and said how he was her favourite player and he gave her an autograph and went to get a coffee with her.
5) There are many outlets to have lunch at but be smart and get out of the arena quickly after play finishes in the day session or else you might have to wait ages to get food - and miss watching the players doing some fun stuff in between sessions.
6) Similar to the above, but in the evenings. Although they have brought forward the evening sessions a bit, a tense 3 set singles match can mean that there is an almighty rush to catch the last train. Don't hang about when the matches are finished (unless you are in a spot where you can try to get an autograph).
PS: Do go to the doubles matches as well - they really appreciate playing in front of larger crowds than they are used to, even though the arena is usually half empty for doubles matches. Anyway, your ticket price includes doubles, so why skip it?