She was at her prime in the 30s and 40s, with Benny Goodman, Lester Young, Artie Shaw, et al. By the 50s her voice was pretty bolloxed, and the performances less consistent. Most of the compilations are of stuff from the 50s, however. Stick to the 30s and 40s material, it's some of the most important in the history of sung pop music. If you find yourself worrying about the recording quality, punch yourself in the head for missing the point.
-- Ian