There are really two schools of thought in play here:
1. As a 22-year veteran still on active duty, I've been dealing with the ARs right-handed geometry for so long that I hardly even notice all of the nagging compromises I've been forced to accept over the years (this was more problematic in the A1 era). That said, when I first learned of Stag's left-handed upper design, I couldn't help but view them as a huge leap forward for oft-maligned 10-percenters like myself. We couldn't have asked for a more useful or better engineered solution, and I applaud Stag for their investment. Every southpaw should probably own one of these simply to underscore the economic viability of the concept.
2. From an operational perspective, of course, it's useful to keep in mind that those who rely upon the AR for their livelihood might still be well-advised to pass on the left-handed alternative. Larry Vickers, most notably among others, has pointed this out many times, and there is no arguing with his logic. Since 99.9% of the ARs you'll encounter out in the real world are going to be set up according to the original recipe -- for right-handed operation -- there is real potential for negative habit transfer/muscle memory if you come to grow too fond of your Stag. That's probably only a concern for those of us who are still wearing a uniform, but it really does warrant some serious thought. If you've ever tried to migrate between a 1911 and a HK P7, for example, you can appreciate how autonomic impulses can kick in when they are least expected.
For my part, I suppose I've got one foot planted on both sides of this particular issue, but if your AR is primarily a range rifle and you aren't headed to war anytime soon, I do think the Stag LH upper is a sure winner.
Chief