I am familiar with the SnagMag but I don't use it. I have a Glock 17 9mm and a Glock 30 .45 cal. When using the SnagMag the magazine will be positioned vertically in your pants pocket. I would think that when you sit in a chair or car it would be uncomfortable because the mag is in your pocket in a near vertical position. I have read several user reviews though where people like them.
I use a Safariland 123 horizontal mag pouch/holder that velcros around your belt. I have one for the Glock 17 and one for the Glock 30 since they are slightly different sizes. Because it is horizontal it is very comfortable and it doesn't look like a magazine holder. I bought them from Amazon for I think about $15 which is less expensive than the SnagMag.
I have one for my HK P30 mag. W/ TF +5 base. I carry it in my left rear pocket with out difficulty. It does require some practice, and what doesn't, early on I had the whole thing come out occasionally when I gripped wrong and got a part of the snag mag also by mistake. With a little practice I solved my problem.