As you can see from Dave's experience, always wear these on the 'buckle side' of your belt - between your buckle and the first belt loop. Waaay less chance of it sliding off. Sorry about your experience, Dave :-(
This 'belt wallet' is a similar item to the Lewis N Clark, Eagle Creek, and Rick Steves' 'Hidden Wallets'. I've used one for years (when I'm not wearing a 'regular' waistbelt-style money belt) and you don't have to wear a belt with them. The Eagle Creek version always used to (don't know about these days) come with a large safety pin (OK - a diaper pin!) so you could pin it to the inside of your waistband, pocket lining, etc. Just stick the pin through the loop, your waistband/pocket lining, back through the loop (if possible) and close the pin. (All of this pinning activity happens on the INSIDE of your pants, not on the outside! It's supposed to be a Hidden Wallet...) This way, you can wear it horizontally or vertically, and at any place on your pants (higher or lower, centered or more to one side). For instance, if I'm going to be spending 4 hours on a train, I'll pin it higher (like through my pant waistband) OR way lower (so it rests on my thigh) to be more comfortable. (Those loops are long so they are very versatile.) This also keeps my passport from getting bent from sitting. If I'll be doing a lot of walking, I'll pin it lower and to one side so it hangs farther down my leg.
And now for TMI - I usually wear my wallet of either sort inside my underwear (or at least well inside the waistband of); if it should ever come loose somehow, my underwear is a last defense against it falling completely out of my clothing. All of the moneybelts that I wear are made of comfortable fabric so being against my skin isn't a problem.
You're welcome for that bit of mental imagery ;-)
So...yes, many of us have lots of happy experience with hidden wallets, just maybe not this particular version.
The best one is the one that you'll wear.