They don't taste like hamburgers, they are just the same roundness and offer some substance in between the bun. Plus you can marinade them nicely. Unless you are talking about some fake-o hamburgers where they grind up mushrooms with some kind of breadcrumbs in which case I don't know. If they did that I am sure it would be awesome, much more awesome than hamburgers themselves (which I think are incredibly boring at best and often yucky).
To the person who asked if vegemite is salty, YES. It is less salty than it was decades ago, they have been slowly reducing the salt content in it over the years. This is a little know fact.. they think we don't notice but I remember!!