The first thing to think about when buying a scanner is what you want to use it for and how often you plan on using it. You might want a scanner for scanning paper (i.e. text or snapshots), film (i.e. slides or negatives), or both. You might need to only scan a few things per week, or you might scan hundreds of things every day.
If you're only occasionally scanning a few things and don't have a need to scan film, then get one of the small, light Canon LiDE scanners. You don't need to plug them into the wall, they're small and light, don't take much space on your desk, are plenty fast, and produce excellent results. They're so inexpensive that pretty soon Canon will pay you to take one.
If you're scanning a lot of paper and you don't need to scan film, then one of the less expensive Epson scanners are a good choice. They're very fast, and will save you a lot of time. Make sure you get one with a USB 2.0 interface.
If you're scanning mainly film (slides or negatives), then the Nikon CoolScan V ED film scanner is an excellent choice. A critical feature when scanning film is the ability to use an infrared lamp to detect dust spots on the film - Kodak calls this "Digital ICE", but it's just an infrared light source. The Nikon scanners use an LED light source for red, green, blue and infrared light, and these LED lights are better at producing pure colors when scanning film.
If you mainly need to scan paper and only occasionally want to scan film, you might look at the Epson Perfection 4490, 4990 or the Canon 9950F. All three have a light source in the scanner lid, and all can scan film with infrared light. All three are also quite fast when scanning paper. The film scans from these scanners won't be as sharp as those using the Nikon CoolScan V ED film scanner, but they'll be sufficient for most uses.
The scanners that are sold today are so good and so inexpensive that it's a good time to buy now.
Ed Hamrick Vuescan