I found the Ouran anime far more entertaining and engaging than the manga, but I I can't stand most shoujo manga styles, so that's probably part of it. If you end up liking it, you can always pick up the manga where the anime left off anyway.
As for Death Note... I'm not sure you can go wrong. I will say that the music in the anime is pretty fantastic at setting the mood, so if you like that, I'd go with the anime. But it also depends what you have time for - manga is faster, and the Death Note manga is very well done as far as storytelling and mood go.
Hmm... I'd say that, !!YEAH!!, it is definetely worth it! I really enjoyed both of those series in both anime and manga form but I guess enjoyment levels on either would be determined by what type of otaku you are because they are both from COMPLETELY different genres. Death Note is more of a shonen manga (boy's manga) with lots of blood and awesome creepy-ness while Host Club is more shojo manga (girl's manga) with obnoxious insanely rich guys. But, based on my experience with these two series I'd watch the anime of Host Club and read the manga of Death note (although the manga for Host Club is also very good).
I have a friend who read the geass manga and said it sucked. Apparently their weren't even any mecchas in it like the anime. And I'm pretty sure the anime came first, so shouldn't the manga try to be closer to it?