If you have a lot of 35 mm film you'd like to convert to digital, a dedicated slide scanner, which also scans film negatives and slides, is usually recommended. However, these standalone devices are going away, especially as the capabilities of flatbed scanners to competently scan transparencies rise. Plustek makes several of these types of scanners, including the Plustek OpticFilm 7400, which has since been discontinued but is available online.
If you have other scanning needs, higher-quality flatbed scanners like the Canon CanoScan 9000F (*Est. $150) or the Epson Perfection V700 Photo (*Est. $540) may make better choices for your film-to-digital conversion. They can batch-scan slides and negatives, generally about four at a time, while also providing higher resolution, and they can be used for other scanning tasks once the slide collection has been transferred.
A dedicated film scanner, in the words of CNET's Lori Grunin, can become "an expensive paperweight." If you only want to scan slides and negatives, however, a scanner such as the Plustek 7400 may make sense.