I handled a 6" one in a local shop. While I didn't fire it, I must admit that it was surprisingly comfortable in the hand and balanced pretty well. Much better, in fact, that I'd think it would based on its odd look.
As for 9x21, Glenn is right that it's popular in contries where "military" cartridges like 9x19 are verboten. Basically, 9x21 is a 9x19 that's had its case lengthened by 2mm but loaded to the same OAL and with nearly identical ballistics. The reason for this is that you get a "non-military" cartridge that is therefore legal, but all that is required to convert a 9x19 semi-auto is a simple barrel swap.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar