I generally play the beta because it's good to help in identifying bugs/balance issues (and see the new features/quests etc) - but after encountering and reporting a couple of the most significant bugs in 1.5.8, I reverted to 1.5.7 (if your broadband is decent, only takes a few minutes) because I was enjoying that game, and the severity of the bugs meant that further 'testing' wasn't really going to be of much value. For example, the hordes of bandits affecting small areas would mean the economy in that area was going to be messed up, which would affect the wider economy, local prosperity, party recruitment and therefore wars; so everything, basically. There's not much point in play-testing something so clearly messed up, because any observations would be distorted by such a significant bug.