Although I am the current secretary of the society, I am very much writing this as a personal perspective. I think I saw a lot as one of the co-organisers of SMBE 2012 in Dublin and maybe this will help somebody make up their mind about what is involved.
First of all, this whole process will take about two years to do and will cost you 3-4 months worth of work. So yes, it is pretty involved and you shouldn't think about doing it unless you have a good bunch of collaborators that you can depend upon when things get busy.
We had an excellent Professional Conference Organiser on the job - MCI Dublin - and we depended on them a lot to shape the event and to negotiate for us. It might sound like they cost a lot of money, but what they saved us in their negotiating skills, extensive list of suppliers and good advice on how to make the meeting work meant that they actually paid for themselves. I calculate that even though they handled the website, abstract submission, dealing with the conference venue, printing out the materials, posters, etc., they cost the conference exactly nothing. If we didn't have them, then we would have had an impossible job, would have had to recruit dozens of people ourselves and the meeting would have cost more (read - it might have made a loss).
We set out a one-year timeline from the end of the meeting the year before (Kyoto, 2011) and we tried - and failed from time to time - to stick to this timeline. The society is a very "Bottom up" society, so the meeting is a collection of symposia that are proposed by the membership. We promised the symposium organisers some money in order to incentivise the speakers and make it easier for the symposium organisers to get good speakers.
The timeline was very important. We asked the membership to submit proposals for symposia. We expected to get about 25 proposals. We got 80. So, GREAT - we are not short of proposals, but Ouch!! This meant that we were suddenly going to be disappointing a whole load of enthusiastic organisers. We did our best to include as broad a diversity of disciplines as possible, but inevitably we got emails from people that were "perplexed", "disappointed", "frustrated" etc. because their symposium was not chosen. We simply couldn't fit a quart into a pint bottle.
The meeting has to balance a few things - if it is too long, then it costs delegates more money in hotel accommodation and we have to rent the convention centre for another day, which puts up the registration fee. If it is too short, then not enough people get to speak and there isn't enough time to see the posters. If we have fewer parallel sessions, then fewer people get to speak. If we have more parallel sessions, then people get frustrated that two "must see" talks are on at the same time. So, you strike a balance and hope it was the right one for the greatest number of people.
The meeting went ahead. Nobody died. We spent €2,000 on water, for instance. €50,000 on lunches. €25,000 on renting poster boards for three days! We discovered at the first break that almost nobody in the society drinks tea and almost everybody drinks coffee. Major coffee shortage ensues for the first break. Gallons of tea go down the drain. A year earlier at a small Irish meeting I ran, almost everybody drank tea in the morning. You just don't know these things in advance.
Overall, the meeting is now a million dollar meeting, between income and expenditure. We just about broke even on the budget - which meant that the last month consisted of day-by-day monitoring of the budget to see how nice should the snacks be at break time, or how many refreshment vouchers can we give out at the various receptions. We cut a few things, but we were pretty safe with the budget in the end.
What did I learn? Well, for a start, you can expect to be treated very badly indeed by a small minority of conference-goers ("you are incompetent", "I will never again attend and SMBE meeting", "Don't you realise how important *topicX* is? Why does it only get three hours of talk-time?", "I am going to have my Darwin-Tree-Of-Life Tattoo lasered off in protest", etc).
This is more than offset by a much larger set of lovely emails you get from others - some that you don't know, as well as those you do know. I felt that the experience told me more about some of my fellow scientists than it did about us, the organisers. Usually, the people that are the biggest complainers are those that have never organised a large meeting before.
It is very difficult to predict the popularity of a symposium. Almost nobody submits mathematical papers to the maths-y symposia, but lots of people go. We got hundreds of submissions to the "early-stage Researcher" symposia, yet the talks were among the poorest-attended. These issues matter when you are paying for rooms according to their size. You try to guess the popular symposia and put them in the biggest rooms, but the best predictor of crowd size has yet to make itself known to me.
Would I do it again? Oh, for sure. It was a BLAST to have 1,300 of your closest friends come to your city for a few days. People still come up to me and talk about it. I would wholeheartedly encourage people with the energy and a small collection of dedicated colleagues nearby to put in a plan. Get your hands on a Professional Conference Organiser and do some talking with them first. Then, if you are still inclined, put in an application and go for it.