As much as I fell in love with the Doctors daughter, I would fear another spin-off series. How awesome to have her as a companion in the future though?
The way she blasted off into space with a big wide grin on her face, circling the planet in a wide arc, just made me think "here's a fiesty female space adventurer". She'd be great as a lone character AND a great role model for young girls. They don't have too many female sci-fi characters. Personally, I think she'd be great, but each spin-off series dilutes the brand and takes a little bit away from Doctor Who.
As for Martha, I agree, what was the point? Unless they are just setting it up for Jenny to meet up with Martha on Earth. That's the only thing I can think of. The bits with Martha outside were really pointless. In the end Martha was responsible for her companions death. She coaxed him out, then was careless and allowed him to die. I felt really ripped off by that bit, despite her sobbing bit.