Capturing your video via Firewire is, indeed, losless. As long as you do not compress or decompress your video, it stays the same as captured on tape. EVERYtime you alter the video (dissolves, color correction: the works) you'll go through a decompression - compression cycle, and you'll lose quality. That's the reason they mention having a cuts only transfer - then you don't lose anything.
If you're planning on heavy duty editing, you could capture via SDI and then stay uncompressed in the rest of the workflow. Even better is to capture straight to disk while shooting - but that's an other story.