Flatbed scanners usually have only a quarter to one half the optical resolution of a dedicated 35mm film scanner, ie. 600 to 1200 dots per inch (dpi). When two figures are given the smaller one is the true optical resolution, the higher one being obtained by having more steps down the page, by controlling the stepper motor.
Therefore, even though many such scanners have an integral or optional 35mm transparency hood, the quality of the resultant images is usually not as good as a dedicated 35mm film scanner. The new Epson and Microtek flatbeds are an exception - see bottom of page. If the transparency hood caters for medium film-format the resultant image can be equal to a 35mm scanner because the image needs less enlargement.
What you must remember is that you can only get a scan as good as the original image placed under the hood. Often prints produced by the average high street film developer, using an automatic processing machine, can leave a lot to be desired. This has happened to me, but by using the 35mm film scanner and the negative I was able to obtain a lot more detail in highlight the areas.
Having said all this a flatbed scanner is a useful accessory to any computer system, as a colour photocopier or for optical text recognition. I recently purchased a Canonscan N1240U flatbed to complement the CanonScan FS4000US 35mm film scanner. This replaced a Umax 2400 USB flatbed scanner which didn't work properly under WindowsXP.
If you cannot afford a dedicated 35mm scanner or want to scan medium format film, then give serious consideration to a optically high-quality flatbed with a transparency hood. For medium format scans, note that some flatbeds only allow scans from 35mm film. However high quality scanners are expensive, it may be a better and cheaper option to choose both a dedicated 35mm scanner AND a cheap flatbed.
Epson, Canon and Microtec now have flatbed scanners on the market which have an optical resolution higher than the Minolta Scan Dual III dedicated film scanner.