The Rode mic is okay for the cash. With any mic, positioning is key. On top of your camera in place of the built in mic is about 10% better.
Close to your subject, even into a recorder is better.
Audio usually means £££'s, but as you've stated that isn't your priority then maybe save the money from the robe and put it wowards one of the cheaper zooms, or tascams, something you can place close, just out of shot, the cheaper zooms (H1) still have fairly directional micing, even if they can't take pro mics, but perspective / placement is equally as important as the mic you use.
Synch the files in post.
Not by any means ideal, but as good as you'll get with your budget, and better than a camera top mic.