2. Good uses for your towel
3. Incessant insults of George W. Bush.
If you go to the movie:
1. Have very low expectations.
2. Stay for the fun bit during the closing credits (you don't have to wait until it finishes).
On the Shockwave ratings scale of 9-23 where 9 is low, 23 is high and all Steve Brust novels get a 17, I rate HHGttG at about a 14.