Good question, I was wondering this as well.

Is there anything wrong with Vista media centre or Windows Media Player 11?
I use VLC player. Just seems to work and it's free. WMP11 crapped out on me not showing the video but playing sound on DVDs etc. So yes, PowerDVD is also good.Used to have it but reinstalled.
ESP Eclipse-II VTB (With 18v Mod)
Randall RG75
Tapco Mix60
New Zealand
I use PowerDVD. It does the job. I don't think any DVD software is worth getting excited over.