I'm still undecided what blog to go with. This is what I've found so far with my limited tinkering...
Tumblr
Pros - customizable, iPhone app, free domain mapping
Cons - relatively new (concerned it may lose funding/disappear)
Posterous
Pros - customizable, domain mapping
Cons - very new, WTF kind of name is Posterous?
Blogger/Blogspot
Pros - very easy to use, customizable, domain mapping, Google owns it
Cons - no iPhone app
Wordpress
Pros - been around for a long time, lots of modules/widgets designed just for WP
Cons - $$$ pay for domain mapping, custom templates and anything else interesting