It is available online from googlevideo (above) at Internet Archive and for sale on DVD.
The movie is dark and bleak and hopeless. Not gonna work for someone whose favorites feature the Marx Brothers.
Detour has been selected to the Library of Congress National Film Registry for preservation but is not on imdb's top-50 noir list.
update 6/8/2007: Bright Lights Film Journal article, reviewing the film and its dvd release. From the article:
Detour has one of the more convoluted plots in noir, packing a flashback structure, an extended voiceover, a cross-country trek, a mysterious death, an "accidental" murder, an identity exchange, an unforgettable femme fatale, and one of the most pathetic, masochistic antiheroes ever into its 67-minute running time.
1001 Flicks has a review.
Movie Zeal is including Detour in its Month of Noir.
FilmFanatic has a review.