"Well, it did require modifying. You had to remove the hardware (I forget the name of the pins--one holds the trigger group and another holds the elevator pivot on a Mossberg I think) and replace them with cheap crap from China. Especially given Tacstar's bottom of the barrel reliability, I would recommend you return to stock."
For what it's worth, Ive owned a Mossberg 500 persuader, Remington 870 Wingmaster, Winchester 1300 Defender and Mossberg 590a1 all with Tacstars installed and have been impressed every time. Keep in mind that's a fairly wide array of slightly different shotguns and haven't had a problem with any of them, and unlike the mallninjas, I actually do go out and shoot the heck out of my guns. I can't vouch for Tacstar's other stuff (and ATI is beyond awful imo), I believe their sidesaddle is a very decent accessory if you have tried one and don't mind the balance difference. I'm sure the Mesa is quite decent, but I'm so impressed with the Tacstar I definitely couldn't justify another 50 dollars on one.