In general, I find the wikipedia maths pages to be very comprehensive and useful, and I have not found any serious errors. However, they are often not very well organized or clearly explained. I agree that it would be good if more professional mathematicians tried to help with this.

I rewrote the article http://en.wikipedia.org/wiki/Homology_theory a few years ago. I thought about working on some other related ones. However, it seemed to me that it would be best to do a general reorganization, deleting some pages, renaming others and redistributing material in a more coherent way. I did not know how to go about presenting such a proposal to the relevant community, and how to ensure that there was enough consensus that a lot of work would not go to waste, and how to make the changes offline so as not to leave an inconsistent mess while the work was going on. So I did not end up doing anything.