I use basecamp. I like basecamp. I like the 37Signal designs. I read their blog. They are certainly pioneer in the newly revitalized ASP + Web2.0 biz. But I also find basecamp inadequate, as a paid user. Specifically:

While it is nice that the include their writeboard function in basecamp, it is not integrated. If you want to email a writeboard, the basecamp user list is not available for email selection.
Their todo list is too simple. To manage a big project, the lack of any type of hierarchy, tagging, or even just TOC makes it hard to use. This is ironic since one of the great thing about 37 signals is their simplicity in design.
Example: I need to be able to move entries between to-do lists, as we use them to classifying tasks, and sometimes tasks get mis-classified.
The login URL is NOT yourname.basecamp.com. and
there is no login button on their website.