[Haskell] Newbie: what are the advantages of Haskell?

Hello, what are the advantages of haskell over semi-functional programming languages such as Perl, Common Lisp, etc.? What are the mysterious "side effects" which are avoided by using Haskell, which everyone talks about? Null pointers? Don't you ever get null pointers in Haskell, including when doing IO? Aren't Haskell's advantages outweighed by its complexity (Monads, etc.) and rigidity? Last but not least, I would like to learn from those among you who are former PERL developers, why you switched to Haskell. Many thanks. phiroc