We just returned from Expo 2010 and were able to get "Golden Tickets" (tickets to the China Pavilion) by lining up at Gate 1 around 6am. We actually arrived on another day at a bit after 7am and received them as well. Unless things change, if you are one of the first 30-40 or so in line, you should get one. The wait in line was not bad, except for right at 7:00 when they open up the security line area there was a moment of people trying to rush the line. Once in the security line area there was no way to cut in line. Many people were sitting on little plastic stools and eating or chatting. That area is covered and there is a bit of cooling mist to keep people comfortable.
They were handing the tickets for the China Pavilion out as you came through the turnstile. Ours were for 2:30-3:30pm. Even with the tickets, we had to wait 90 minutes or so in line before we actually entered the pavilion. As with most of the pavilions, it was interesting, but not all sure it was worth the hassle. (Wish we could have passed on our second pair of tickets.)
We spent four day at the fair, but missed seeing many of the national pavilions due to lines. Crowds were up to the 500,000 level for some of the days we were in Shanghai (up to 400,000 one day we were at the Expo). We did enjoy spending much of our last day on the Puxi side, where things were a little quieter, especially in the Case Building area.
The Expo was definitely worth going to. The exteriors of the pavilions are more interesting that at Aichi and are eye-catching all lit up at night. Don't feel like you have to see all of the pavilions to make it worthwhile. For many of them we thought afterwards that they really wasn't worth the time we waited in line.
We were told that hotels are pretty well booked in Shanghai. It is pretty easy to take the metro and/or a taxi to the fairgrounds, so I think anywhere you can find a descent place and price should work.