Their approach is to integrate tools like Trac and use a single
login/password to access all the different components. So when you
create an assembla "Team Space", you know each person in the members
list can edit the wiki and checkout from svn using the same password.
It even has a time-tracking function to support hourly billing, but I
haven't quite figured out how that works.
One tradeoff is that Assembla is fairly generic -- it gives you a
bunch of basic tools, and they run reliably, but probably aren't going
to specifically support your development process or match your company
colors and fonts. Another may be that it's a hosted solution -- not
something you can easily hack additional features on to.
shimon.