Extra round mag capacity BUT you would have to work out the kinks.....
First off, the mag used to drop so I contacted the manufacture & they exchanged it for another one, that problem got fixed. The Next one, I had to file & slightly bend the front edge of the feed lip? (not sure the exact name of the metal under the follower). The spring was gritty & difficult to load rounds 8 & 9 even with a maglula BUT I swapped the spring & the follower with the Smith and Wesson Original. So far by hand cycling it, it cycles & ejects all 9 + 1 rounds, I have not had a chance to fire it at the range yet as of 7/17/2016. Overall, its a nice spare magazine to have, if you're not scared to work out the kinks then go for it.