I did a lot of research on the best poker rooms in Vegas in advance of a recent trip. I really only play NL Hold'Em tournaments, and my research revealed that Aria & The Venetian have the best tournaments in terms of overall structure (i.e. starting stack, blind progression, doesn't soon turn into bingo). These casinos are also high-end ones, meaning the poker rooms would be nice and the ventilation would be good.
In all of the above respects, I found the Aria to meet these standards. I was very impressed with how nice the poker room is. The structure was as good as advertised. You could play your game without being squeezed into all-in bingo if you get cold cards the first 1-2 hours. Most of the dealers were professional, but one or two were a bit off. Overall, I would say Harrah's has better dealers.
I played in about four tournies the week I was there, doing rather well and in fact making the final table in two of those tournies. The tournament director was a very professional lady, and ran things very well at the final table, perfectly calculating the chop amounts and ensuring timing was taken care of. However, the biggest curveball for me was some of the other players. Both times I made it to the final table, once we got down to the final 5 players or so, I thought I was playing a game in the back room of a biker bar. Loud, large, drunk men making one obnoxious comment after another. I even witnessed one player make an anti-Semitic comment to another player after losing a hand. The dealer(s) did absolutely nothing. I can't really fault the Aria for this too much (players show up and they have to seat them), but it was just strange to be in this elegant, luxury casino with a $140 tournament buy-in and be surrounded by tank-top wearing thugs hurling alcohol-fueled vitriol at each other. But this is part of playing poker. It's gambling after all.
That being said, I found other places to be a bit more of a relaxed game and just felt more comfortable there (e.g. Harrah's). Even though I didn't win as much at those places as I did at Aria, none of this is life-changing money, and I'd rather not spend 6-8 hours with some of the people I encountered at Aria. I may have just had some bad luck that week (people-wise) but for such a nice casino, this was a surprise and not the experience or atmosphere I anticipated.