I've just had my final exam for university extension biology, and if you are considering doing it then just keep in mind that it is definitely not a breeze. I loved semester one, which pretty much covered all of 1 2 3 4 units VCE, but semester 2 was much more challenging... a step up from semester one with more complex genetics and classifications/life over all evolutionary history.
Don't get me wrong, i loved the subject as a whole!
Also, check out the vcaa website because they are rules of the course for next year in terms of all uni subjects to be included as part of a vce load. The increments don't quite work the same. I haven't really had a look at it, not having to worry about vce from next week onwards (WOO!).
But check it out

It was definitely worth it this year because I got a foot in the door at the uni that I want to go to (melbourne) and I have an increment which is much higher than one of my bottom two subject would have been
Also, make sure you consider your course load. The uni subject is very much another VCE subject, and you have to give it that sort of attention to do well. I had to drop a subject so i could keep my biology there. It's a lot to think about, but good luck! Just take everything into account when making the decision.