Basecamp offers excellent service and user support; they release a regular stream of updates and the user interface is intuitive enough that even my most technology apprehensive clients have an easy enough time using it.
Instead of maintaining a single development trunk, Basecamp has a problem with releasing major updates as entirely new, yet mutually exclusive, platforms. As a result, migrating projects is a nightmare and with every new major release comes an entirely new pricing structure. Those new pricing structures always favor large users; meaning, if you need less than a few dozen projects, you end up getting stuck with a 30 percent or more price hike. At this point Basecamp 3's entry level price point is unrealistic.
If you are a large company or provider, Basecamp will be fine. For everyone else, you're out of luck.
Basecamp is good for coordinating client/provider communication and the integration with email client response functionality makes clients more apt to use it.