Most slide viewers are pretty cheap. A good loupe will let you see what's on the slide more clearly and won't cause eye strain with prolonged viewing. Cheap loupes, like those plastic Agfa-type 8x loupes, are prone to heavy pincushion distortion. A current model Schneider 4X will show you more than one of those 8x loupes, and it's good for 35mm slides and contact sheets as well as for groundglass focusing. For slides you want an opaque base, and for contacts you want a clear or translucent base. On some loupes the base is interchangeable.

You might also want a high quality 6x, 8x, or 10x loupe to assess sharpness or enlargability.

The latest Schneider and Rodenstock are generally the best, but Peak, Mamiya, and Kaiser also offer good loupes.