This is a fine product, plenty of room, sits securely around your waist, and would make it extremely difficult for someone to swipe your stuff without you noticing. Where my qualms come in are when it comes to actually being "undercover."
As I said above, the wallet works great for keeping your stuff out of reach, but it's not very concealable unless you're wearing a jacket or something baggy and dark colored. I spend a lot of time traveling around 3rd world countries and sometimes I have quite a large sum of cash on my person as I'm leading groups of people around. We're already a target because we're usually the only white people around. I don't want throw another one on my back by inadvertently advertising to an assailant that I might have a large bit of money that I'm trying to hide from them. And seeing as most of the places I'm visiting are not the type where you're going to be needing a jacket, I found it hard to conceal this wallet when I tried it on at home.
Again, it's a fine product, and if you've got the ability to conceal it under baggy dark clothes or multiple layers then it will work fine. My problems are that it's hard to conceal when you don't meet the conditions above.