Dreamhaven is fine, but hardly best sci-fi bookstore. That is and always been Uncle Hugo's. Uncle Hugo's was better when Scott Ames was still alive, but even with his passing all those years ago, it is still the best sci-fi shop in the US, not just the Twin Cities (and probably the world).
BEST SCIENCE-FICTION BOOKSTORE - 2010
Dreamhaven Books & Comics
After decades of operation, multiple locations, and changes in emphasis, Dreamhaven soldiers on. The key to the bookstore's continued survival—no, evolution—is owner Greg Ketter's ability to follow his own passions and let the audience for those passions find him. As it always has, Dreamhaven caters to the genre fan, from racks of recent science-fiction and fantasy paperbacks to limited-edition hardcovers to rarities past and present. It doesn't hurt that Dreamhaven is Neil Gaiman's sort-of unofficial home, with plenty of signed copies and other rarities from the author. Though his store is mainly a one-man operation now, Ketter knows his science fiction (and horror, and pulp, and comics...) making an in-person visit always worthwhile. If you're not feeling social, an extensive website will help you unearth finds like that rare A.E. Van Vogt book you just have to have.