OK, I ranted a while ago about how if you want the "best AR15" out there, you should get a Colt, BCM, Spikes, LMT, etc... It's true, these manufacturers make their weapons to a higher standard. That standard is explained in more detail in this link. Reading it may be beneficial to understanding where the Stag is falling short and why it might make a difference.
However, of all of the manufacturers out there that don't make the cut on the infamous "AR15 chart" (meaning, they fall short in many of the milspec requirements), Stag is easily my favorite, and the best value in my experience.
I have owned several personally, but I have also worked on, shot, inspected, sold, and/or used countless Stag parts over the course of their existence. I have had plenty of chances to compare them to various other makes/brands as well as to each other over the course of the years. I am quite familiar with their quality. I have long believed they are one of the better values out there when one is considering price and quality.
Stag is the "house brand" you could say for CMT (Continental Machine and Tool), who works as a subcontractor making MANY parts for a variety of AR15 companies, including many who are held in high regard among the high speed low drag community. CMT has been in the game for decades. They know what they are doing.
For starters, Stag Arms complete lowers are as good as ANYONE's! I mean that. You need a forged 7075-T6 aircraft aluminum alloy, hard-coat anodized lower (within proper spec) and at the right price and Stag fits the bill. This is not something Stag is unique in. These days, ALMOST everyone is providing a good quality stripped lower.
However, it's not just their stripped lowers. Their lower parts kits (LPK's), buffer tube/buttstock, etc... are all as milspec as it gets in the commercial US market, and tend to be priced very well. I have used mostly Stag Arms lowers, LPK's, and complete buttstock assemblies on most of my personal builds for this very reason. These parts from Stag are good to go. I have experience with dozens of these. Stag is my preferred LPK. They just always seem to be well within spec.
They don't sell their complete lowers with H Buffers, but you can get them from Stag, and I have.
Some people like to gripe about the deer head. I couldn't care less. I don't sit there and stare at the roll mark. I just need something that works. I have owned several of their lowers (still do). Never had a problem with them at all.
I've often said, to get the best quality at the smallest expense, one should get a complete Stag Lower and mate it with the complete BCM or Spikes upper of their choice. There's nothing wrong with doing that. I've done that or something like that with countless builds for me and when helping friends/family assemble their own AR15 build.
With that said, their uppers aren't terrible either. Their 16" uppers have been reliable for me. I've owned and shot with several throughout the years personally with little if any problems. One of my AR15 uppers from them was taken on a carbine course and ran about 1500 rounds through it in a matter of two days and no cleaning. Never had one issue with it in the class, it ran like a champ without one failure and just a little lube.
Stag carbine uppers (16" barrel) have 1/9 twist chrome lined barrels and an individually shot peened and batch MPI inspected bolt, black extractor spring insert. They also use an AR15 bolt carrier. You may want to check the carrier key staking, as that is an easy fix if it wasn't done properly and could save you a small headache.
Unfortunately, they lack M4 feedramps. Not a huge deal, but it costs nothing to do it so I don't know why they don't.
They use a lower quality barrel steel than is required for milspec. However, I don't think this is a big deal either. Most people will never treat their rifles quite like a machine gun. It is still a quality firearm grade steel and most people will NEVER know the difference between 4150 MIL-B-11595E barrel steel and 4140.
With the few small issues I have had with MANY Stag Arms, they have always been good about making things right in short order as they have excellent customer service.
My biggest gripe when it comes to Stag Arms is their lack of variety. I'd love to see a 14.5" barrel from them or a mid-length 16", but it is what it is. However, I think they do well filling a space in the AR15 market for a decent quality rifle at an affordable price.
They make a lefty AR15 upper, and sell many ambi controls, and they also sell a piston driven upper. I don't have any experience with the Stag piston upper (I believe the piston is a gimmick personally), so I really cannot comment on how good it is, but I have heard good things about it.
*This information provided is about Stag Arms standard rifles that they market and sell. They have just started to sell a "Plus Package" that will meet the milspec requirements we have talked about. For only $100 more, you can get a complete Stag Upper that meets milspec requirements. Not a bad option.