I was attending ICM 2002 in Beijing, in my first postdoc year, which was wonderfully organized. I expected that as a general mathematics meeting, the talks should be very accessible, but even plenary talks (with few exceptions, like the talks by Mumford and Hopkins) were accessible (for nonexperts) only about first 15 minutes or so, depending on your attention stamina in the midst of flood of data and the background. My background is reasonably wide, and I am used to find myself well at conferences in a number of area, but most of the talks at ICM were too detailed and fast. Some plenary talks had of the order of 45 transparencies. During the breaks people run in other lecture rooms, and in the mass of people people get lost around. I would recommend knowing and contacting in advance some people you like to hang out, improvising is more difficult at such big conferences than in small ones, which are mathematically good for me. It also depends on how far is the hotel from the site.
India is a wonderful country, which I visited in 2007; the summer is a bit too hot though Hyderabad is slightly in the hilly and drier area than big cities like Madras and Kolkatta; in India hygiene is often a problem (and much worse than in China), especially with vegetables and water (UV-filtered water which they use is OK, you do not need bottled water though). I brought a bottle of home made brandy and was taking a sip few times a day for precaution and never had a stomach problem, unlike most westerners.
I have math/physics friends in India, and if Croatian science funding were not in crisis (and still declining) I would go. I do not know your background, but if you come unprepared, without good acquaintances and so on, be aware of hectic atmosphere of big congresses and the needs of the climate, hygienic and other adaptation in subtropics.
