I've got mixed feelings about it. It is annoying sometimes that your opponent knows what your team is and can predict better, but you can also predict against them. However, I enjoyed the metagame much more when there wasn't team preview (4th gen). I believe the PO server took out Team Preview in Dream World OU (last time I played, at least. maybe it's back?), and I really enjoyed playing much more. It actually brought back the necessity for a lead role on your team.
But with that said, I do like it sometimes so I can predict more easily and can start with a lead that might be good against my opponent. I agree with dragonexpert that the 5th gen team preview helps a lot of newer players predict. So yeah, I'm not really sure if I like it. It's has pros and cons, but if I had to choose, I would prefer a metagame without team preview.