Its surprising how often this topic comes up.
I think last similar one was:
http://giftedissues.davidsongifted....6/Re_AoPS_Book_vs_Course.html#Post200496I wrote our second grade experience down here:
http://giftedissues.davidsongifted....ar_in_math_a_retrospecti.html#Post194683I tend to think about what to do from the end goal perspective. Finishing algebra before middle school is not necessarily that useful for even most smart kids if they are only aiming to reach calculus. Reasons to do so anyway I've considered:
1. Early matriculation to college. In which case you want to make sure along the way that the material is retained especially if done years prior to pertinent tests etc.
2. Interest in math competitions later on. In which case you're looking for depth along the way rather than just breadth and acceleration.
3. Genuine passion for the subject perhaps leading to advanced studies later on in which case you really want to foster mathematical interest and skills and not burn out.
The problem for us at least is determining what my son will want 10 years from now is hard. So for me it mostly falls into keeping our options open as long as the risk vs. reward is not too great.
Good luck
