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We've had this discussion many times, and there are proponents on both sides. You can find some of those by doing a site search (top of page), they make an interesting read.
From a user perspective, the underscores can easily get "lost" in an underlined link and be mistaken for a blank space, and if the domain name is hyphenated using hyphens lends a certain consistency to the look.
If you'll do searches for phrases using two words, hyphenated and underscored, it's an interesting study in the differences in results.
Bottom line, use the hyphens over the underscores, but if you want some fun, space characters work as well!
The way that I understand this problem from past post is as follows.
Google reads the _ as part of the word, and reads - as a space.
red-blue is a 7 letter word
red_blue will be 8 letter word.
But from this if you do a search for red-blue, red blue, red-blue or redblue, the results are always different.
Why not just build these pages as - and _?