Thanks DJ Lein. :)
I use Producteev to manage multiple projects from different organizations to personal and I have been enjoying it so far, especially in conjunction with Google Docs (they should do an integration!).
I find that using workspaces for different businesses and tagging by project works well enough for me, as I take a general GTD approach to tasks — but I think that larger projects that have multiple facets may be better suited to a workspace. It really depends on how visual and compartmental you are — and how many different people you work with.
If you need to see each project separately and manage tasks and subtasks by label, for example, I would take advantage of the unlimited workspaces.
Bear in mind, though, unlimited workspaces only means you can have as many as you want — if you need paid features on many projects that are related, you are better off financially to use one workspace and implement a seriously awesome tagging system for organization.
Hope that helps! :D