X Products have been in the drum magazine game for a few years now. They offer 50 round drums for 5.56x45 NATO platforms such as the AR, SIG556 and others as well as 9mm, and 7.62x51 NATO for platforms like the M1A, DPMS SR-25, AR-10, G3/PTR91, FAL, and SCAR17. X Products also goes as far to offer a skeletonized drum as well.
The compact aspects of these magazines, as well as the increased ammunition capacity offers an equal trade off between tactical advantage and firepower. Having the ability to put 50 rounds down range without swapping magazines is surely a positive, however like all drum magazines the side profile of the weapon system increases along with weight.
We have yet to test any of the X Products drum magazines, but we're working on picking a couple up in the future (likely 5.56x45 NATO-AR15 and 7.62x51-M1A versions).