If a microphone helps me learn about how other mics in my collection sound, then it is worth spending time with it for this reasons alone. The KEL Audio HM-2d not only accomplishes this feat, but does so at an incredibly reasonable price.
It's well understood that condensers generally have more accurate transient response than dynamic mics, and same goes for typical condensers exhibiting more "air" at high frequencies than typical dynamic mics. What quickly surprised me about the HM-2d is that because of its intentional design, it rolls off air and emphasizes midrange fullness, and therefore, the transient detail seemed less pronounced. While closer inspection revealed the transient accuracy that I expect from condensers, the mic's unusual frequency-response curve had initially masked this fact. I suddenly realized that some of my perception of transient detail stems from focusing only on frequencies at which I notice it.
This unusual frequency response also can help place sounds in mixes. For a folk trio whose acoustic instruments all had rich high-frequency content, using the HM-2d alongside the brighter mics on other instruments helped to give immediate space to the mix before I had applied any equalization. When recording a complex electronic setup with harsh high-end, the HM-2d helped to de-emphasize the distorted high frequencies without losing any detail.
Following the HM-1 (Tape Op #63), KEL Audio has again released an unusual and reliable microphone at a very sensible price. While designed to complement a budget microphone collection, its atypical sound can prove quite helpful, even when more expensive options are available. ($179 direct; www.kelaudio.com)
Tape Op is a bi-monthly magazine devoted to the art of record making.