I have a feeling stacking sentries, (especially against criminals), is going to become a somewhat common practice, and quickly.
The efficiency this killer brings is rivaled only by the likes of Garrote, Faerie, Alias, and Switchblade/Dagger. All of which carry some hefty drawbacks. (Expensive and 2 memory, trash-to-use, centrals only, and stealth credits, respectively.) And while Mongoose has it's own obvious drawback, it seems a lot less restrictive than most of the others. It's also fairly cheap to install, and while slightly inflexible with pumping up strength, breaking 2 sentry subs for a single credit is almost unheard of. (Unless you run what I feel is its only real rival, GS Shrike M2)
At only 2 influence, this is a pretty easy splash, especially if you're packing something else to take care of any stacked sentries, like an AI or the soon-to-be-tested-everywhere Panchatantra + Gingerbread/Deus X/Sharpshooter/whatever.
One interesting note, it IS possible to use a single copy of Mongoose on multiple sentries on a run, if you're willing to pull some slightly gnarly jank to do it. Trash it mid-run either by installing another program at instant-speed, or with something like Heartbeat. Then re-install it with a Clone Chip. Since the game considers it to be a "new" install of Mongoose, you can then break another sentry.
(Also, the most obvious counter-play to Mongoose, (stacking sentries), amusingly makes the arguably-worst-killer-of-all-time, Pipeline, ever so slightly more playable, due to the fact that pipeline keeps its strength through the run.)