Though features-wise activecollab has been seen giving basecamp a run for it’s money, the area where it clearly lacks is usability.
This point becomes clear if you just compare the dashboards of the two apps.
Basecamp is so intutive that rarely do I have to look around to find some information. And even the effort required to digest the displayed information is minimal in basecamp. The fonts and visual clues used on basecamp are much better than those in activeCollab.
Another feature which I do not like about AC is that it just sends links in emails and not the actual messages. Basecamp sends the actual messages in the email notification, hence avoiding the unnecessary trip to the project website.
I agree that AC is under heavy development but definitely it needs a lot of love.
I find myself much more productive on projects which I run on basecamp that I plan to stick with basecamp and am happy paying them my money.