I have tried several apps including Goodreader, and I have found none render all my PowerPoints accurately. I use PowerPoint 2007 on a PC, and for some reason (I wonder why? hmmmmm) The iPhone/iPad have NEVER been able to accurately render them. It messes up bullets, graphics, formatting, ughhh.
For now the best I have been able to do is PDF them and use Goodreader or PDF Reader Pro. I have yet to try the VGA out adapter, which would be the last piece to make this a viable presentation tool.
I would imagine a simple PowerPoint would render fine, but I believe the more complex your PowerPoints, the less likely the current crop of options will render them effectively. If Microsoft was smart they'd put a team together and build a PPT viewer for the iPad. And a Live Messenger client. It would be smarter business than this constant war with Apple... 🙄