A client was interested in the Business Data Catalog, so I finally had a chance to explore it in depth, and I came away impressed. Once it's set up correctly, it is amazingly easy to integrate data from your SQL databases in to SharePoint lists, libraries, and search without duplication.
I've spent a lot of time and effort doing similar things for clients who are not running the Enterprise edition, and so can't use the BDC or Forms Services, and I now know the BDC is definitely the way to go.
A couple observations:
For phase 1 of a BDC implementation, focus on using Views from the database rather than trying to set up complex associations in the application definition (XML). That's much easier and you get 90% of the value quickly (in days, literally).
Configure search with a scope for Business Data as part of phase 1. Make this feature very visible to users (shout from the mountain top -- this is cool).
Buy a copy of Meta Man for phase 2. It solves the association problem.
Implement your custom actions in SharePoint using the Shared Services pages, then export the definition if you need to add entities with Meta Man or some other tool. You can export/import as much as you like as long as you increment the version of the application definition.
Add actions to run the business apps that are the source of the BDC data. This is easy and it adds a lot of value.