I'd agree - those cheap ones are rubbish.
If its just for 35mm slides/negatives, the Plustek OpticFilm scanners are good. I think mine is the Opticfilm 7600i, there's probably several newer models by now. Costs about £200. It does give decent quality scans.
Though it is rather slow - if you have a large archive of negatives, it will take a long time to scan them all...