I know lots of junior people (post-doc level and beyond) who want to organize a symposium or session at their national meetings. There is significant variation amongst disciplines as to the cost and benefit of doing this. But some aspects are common across disciplines.
There are pros. Yes, there are. One, you are visible. The visibility is a plus, especially if the symposium is a scientific success. Two, You get to talk to Big Dogs and ask them to participate in your humble little affair. Three, You will get to hear people who you think are important to your future directions. Four, You may get to help foster those future directions in a somewhat bigger way. Five, you will get to know more people. Six, it will feel very very good as it happens and immediately afterwards: that warm glow of science well done.
There are also cons. Yes, lots of them. One, it is a huge amount of work. Do not kid yourself. It is a huge amount of work. Usually there are two steps – submitting a proposal, and then organizing it if the proposal is accepted. It takes time that could be better spent collecting, analyzing or publishing data. Two, it will take lots of your time, time that could be better spent collecting, analyzing or publishing data. Three, did I mention how much time it will take? Did I mention that as a junior person your priority should be collecting data and publishing papers?
In my view, the rewards are not worth the costs. By all means, if you’ve got the 10-20 first authored pubs, and a big grant, then go for it. Try and enlist a supportive more senior person to help (but keep your name first, and do the damn work yourself). Otherwise, having organized a symposium will not sufficiently enhance a CV with too few pubs. You will feel important while you do it, and when it happens, but 6 – 18 months later, no one will remember.
If, however, someone asks you to be in their symposium/invited to-do, jump on it. With both feet and grab on tight. Put energy into bringing your A++ talk (with garlands) to the show.
Should you decide that you are going to organize one here is something you should not do: Invite people and then un-invite them. This happened to me a while back. An enterprising young faculty, with support of senior person, sent a weird invite to me and 3 other people for one slot in an already organized symposium. Sometimes it happens that someone drops out, and you have to ask others, and they are clearly not your first choice. But inviting three of us (just to see who answers)? That is tacky. The two junior of the other three immediately said yes (and replied to all, so I could tell). Then two days later came an email that said “so sorry, the original person agreed to do it after all, but why don’t we try and organize and propose a 2nd related symposium?”. Crap, they don’t even know if their first symposium will be accepted (this is for SfN, where there is a significant rejection rate. Now, that junior person has irritated four people. I think it quite likely that I would ever want to work with them in the future.