What to expect from attending an ICM? Since ICM 2010 is due to be held in Hyderabad next august, and since I never attended such an event, I am wondering if those MO users who did attend at least one ICM could share some of their wisdom (the normal price registration period ends in may, so I'd like to plan things before then if at all).
What's for sure is that the list of speakers, both plenary and sectional, is mouth-watering to say the least.


*

*is an ICM a good place to learn some new math, including outside one's main speciality?


*what about the "meeting new people" aspect, is the setting making discussions easy?

At the secret blogging seminar the topic has been discussed at length, and two kind of quite different answers emerged (intended to mature-enough grad students and young postdoc):
a. the math talks are okay, sometimes great, but forget about meeting new folks
b. some math talks are truly inspiring, and you get a real opportunity to meet experts you didn't know yet
I'd like to have a wider sample of opinions, or at least know which between a and b is the opinion of the majority, many thanks.
(Note to moderators: ideally this question should be made community wiki, but I don't know how to do that.)
 A: When I attended the congress in Berkeley...
I met people that I knew before but had not seen for a long time:  those I was in graduate school with, for example.
I heard famous big-shot mathematicians deliver plenary addresses, mostly over my head.  But some were great.
I attended sessions dealing with my interests, and heard about new developments.
I had lunch with my publisher and arranged a book deal.
A: 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. 
A: Regarding 1, ICM's are a place to acquire new ideas.  With rare exception, there will be no "Introduction to ..." type material, just "Motivation for..." and "Teaser to ..." and
"Of course, since we all know ..., the following will be obvious ...", which is true for me after a few pages and perhaps months of study.  However, even the last type may contain an idea I can use, even if I don't know what "we all know".
Regarding 2, there are times, places, and opportunities to meet; see below.
I hope one day to compare the lists of attendees from all the ICMs since the one in Berkeley in 1986.  I know I am in all of them, and I suspect that I share that distinction with at most 10 other people.
The one in 1986 convinced me to go to Berkeley for graduate school;  the others have given me a quick exposure to various fields in mathematics and usually too much to think about.  Your mileage may vary, but I found the following routine helpful to me.
1) Each night, plan your lecture tour for the following day.  You will need to get a list of programme changes (usually needed only for Short Communications sessions), and select your A, B and C list choices for each hour. (Sometimes your A choice and B choice get rescheduled or canceled.) If you have friends attending and want to split up to cover more lectures, plan with them.  At the first few conferences, I had mild sessions of agony because there were more lectures I wanted to attend than slots available.  I now have restricted my interests in the afternoons to three or four sections (currently logic, algebra, computing, and combinatorics), to make my planning easier.
2) When you suffer from lecture burnout, try a tour or even just walk around in the part of the city near the conference hall.  One of my treats from the 2006 ICM was looking at the inspired architecture in that part of Madrid.  Or check out the array of posters.
(Ideally, ask permission before photographing a poster.  In any case, don;'t use the
photo beyond personal use without arrangments with the author of the poster.)
3) If you are very social (I was not), figure out how to work that into your visit.  Lunch breaks and afternoon breaks are the most opportune for this.  I did manage to chat up a few attendees (including Tim Gowers and Greg Kuperberg) during moments in between  lectures.  There will be a few social occasions, and in most cases informal dress is acceptable (but check the invitations).  During the
lecture sessions, there will be limited or no time for questions, so make your question count.  Also, unless you find someone to walk to the same lecture with you, it is not likely that you will have much time to chat between lectures.  
4) Find the information booth and help desk.  If any problems arise, they are your first step towards resolution, even if it is visit related and not conference related.  Also,
avoid the temptation to check your email during the conference.  You can easily get sucked in to spending too much time on the computer.  (You get to choose how much time to devote to computer versus sleep, however.)
Regarding your a and b choices, I would say both are true.  In point of a, if I had made a persistent ass of myself, I might have gotten more than a few words with Tim Gowers, but as he had been awarded the Fields Medal recently, I chose to think his time with me would be very limited and in high demand.  Thus I did not persist, and being in lecture-attending mode most of the time, I often have the perception that there is little to no time for socializing.
Towards point b., when I relax, I find I can talk to a number of people and create opportunities for socializing quite easily, if I put my focus on it.  There will be some inspiring talks, and after hearing a few of these, you can use one or more points from them as icebreakers.  I will thus say it depends on what kind of experience you will want to make of your ICM.
Gerhard "Collectors edition ICM T-shirts for sale" Paseman, 2010.04.08
