I'll admit I didn't find it 'funny', but the silliness was pretty good, and didn't look stupid, as with many silly movies. I thought the singing angels were epic, and the Titanic theme song part was funny. The way the characters interacted - supposedly spanning a great number of centuries - was also great to watch, and the special effects were impeccable. Abe Lincoln's part, I found hilarious, especially at the poke at Stiller's height. :P
Gotta love the little bobble-head Einsteins. Epic. :D Watch out for Al Capone and his gray-scale look. That's some cool special effects, I thought.
Definitely worth a watch.