The first season was bloody good. Obviously amped up and over-the-top in terms of camera, music and acting, but it comes together quite well as long as you don't expect it to be something it's not.
Season 2 was nowhere near as good unfortunately. It's not a true prequel as it's completely different people and different city too I think, but there are a few links to what would happen 'later' in the first season with a couple of little cameos thrown in. Still watchable, but to some extent you're watching it just to watch it, not enjoying it and looking forward to next weeks episode.
They are making a third series as well. Hopefully it'll be as good as the first one, but I doubt it to be honest.
It's interesting to see how many of the people that were in Underbelly (mostly the first season but also a few from the second) that have gone on to star in new Australian shows. Rush has quite a few, and it's my favourite Australian show for a long time, and arguably the best cop show out there. It keeps itself interesting, but it just feels more 'realistic' than any other crime show. Rescue Special Ops also has a few people from Underbelly.