Hi,
I've just done a data migration into Alfresco of about 4G worth of documents. I agree that loading through CIFS and WebDav is slow, but if you do it on the server rather than on a client machine, it might be ok. I found the fastest way was to zip a number of files, copy the .zip over through CIFS, then unzip it on the CIFS drive.
I have also used ACPs (Alfresco Content Packages) to load documents that had metadata associated with it (the metadata was stored in an Excel file). ACPs are just like zip files, although I dont think they are compressed. You can load them through the Admin console > Import. They are also what you get if you do Admin console > Export. There is a wiki page on this if you search for "ACP". What we did was write a custom Java app that would create the acp and an xml with all of the metadata attached. Then when we loaded it into Alfresco, we got all the documents pre-populated with metadata.