I've been using Basecamp for about 6 months now and I love it. It does have it limitations but here are my findings for now:
Basecamp
- able to have private conversations between you and your subbies. You just need to add them in under your company.
- able to sort files and messages into categories. Particularly helpful with grouping passwords together etc.
- Can't assign a due date to tasks
- Can subscribe to the feed via RSS or Ical
- Can use milestones to record due dates but its a pain and I usually forget to put them in
- Have a writeboard feature which can be used to collaborate on writing projects. You can see each version and see who has added what.
- Can integrate with their chatting software and chat in real time through basecamp.
Client Spot
- Can add due dates to tasks
- Cheaper then Basecamp when you start to add the number of open projects I have (any given time about 30, with some clients having more than one)
- Less robust but has an excellent timing feature that you can export for QuickBooks (I can't because I'm in Australia and our QB is way way different to the US one - even looks different)
- No chatting
Overall findings
I think I'll stay with Basecamp. I have tried and tested so many that I could possibly review more but won't, otherwise I'll go a bit

ut:
The only other software out there that would make me change is Central Desktop. It is more project management focussed and can integrate with their webinar software. That would be COOL! Price is comparable with Basecamp.
HTH