I'm not completely unfamiliar with them, as I had some in the past but they were more "field" style, as I wore them over my clothes to carry an 8" .44 Magnum when I was OCing in the middle of nowhere.

Lately I've been thinking about giving one a try for EDC especially for long road trips as I just cannot STAND sitting in a car with a gun on my waist.

I see some of the popular ones do not attach to your belt which I think would be kind of necessary to keep things from flopping around.  That is the type I had used in the field previously.

I know these aren't what most people use but I was looking to see if anyone else is out there shoulder carrying---what are you using, any tips, what works, what doesn't, etc.

I like the idea of a more comfortable carry with 2 mags and the ability to conceal larger guns with the only real sacrifice being having to wear Hawaiian shirts or button-ups or whatever.

Andrews Leather Monarch is fantastic. It's not cheap but the harness does not need tie downs. Carries a heavy auto very well, I used it with a Smith and Wesson 1006.

Cheaper is the wrong way to go for sure on this one.  What kind of vehicle do you drive?  What holster do you have that is so uncomfortable to carry in a car?  Is this a body shape issue?  What belt are you using?

I used to hate carrying in a car and it was my belt that was the problem.  I changed to a belt that is infinitely adjustable so I can loosen when I sit down and tighten when I get up.  I found belts that are tensioned off a holes in the belt suck and belts that are infinitely adjustable were just awesome.  Currently I am using an Ares Gear Aegis that works like a Boy Scout belt that you can adjust the tension very easily and are not restricted to the holes in the belt.

We have 3 vehicles:  BMW X3, BMW 335i, and my beater, '03 Ford Explorer.

Maybe it is a body shape issue, but no matter which OWB/IWB holster I use I end up "sitting" on the gun or it is in such a place that I cannot fully relax.  I'm 6'2" with an endomorph body style and I have no ass and a 34" waist, so my pants are always really falling down with a gun as well.  All my belts are tensioned from holes in the belt but I have some old nylon infinitely adjustable ones from when I was in the Army that I can try.

I definitely want to still give shoulder carry a serious look though.  I've never been able to get comfortable with any kind of waist area carry (fine standing, just not sitting) so most of the time I have done pocket carry or just taken my gun out and placed it in the car when getting in.

Shoulder holsters are a sort of acquired taste. I suggest Ritchie gun leather expensive but worth every penny. You may want to also consider a cross draw

I am 6'1" 150# 32 waist with my carry setup so I am a slender fellow.  I found that my belt was the contributing factor to conform in my car (CTS Caddy).  Usually when I sit down I release the tension on the belt and it is quite comfortable with a G-Code Incog for my G19 and XC1 with a spare mag on the opposite side.  I have done long trips +5hr drives and so long as I can release the belt tension I am comfortable.

The mil or boy scout belt is exactly the buckle type I am talking about with no hole and pin and static tension they are terrible for comfort while carrying.

check out the galco classic lite. You can find it most places around $75 and its a great quality leather rig.
stay away from the nylon ones those done have any give to them and dont conform to your shoulders like leather.
it only carries your pistol plus 1 spare mag.
ive worn mine alot and it just keeps getting more and more comfortable

The Knull SMZ carries a Mac11 or Beretta 92R upside down and hides under a tailored jacket.
He has a couple others a little outside the box designs as well.

http://www.klnullholsters.com/default.htm

The galco vertical shoulder holster is OK. To floppy for EDC but is a nice piece to grab that has 2 spare mags.

I had an engraved el paso saddlery spider holster rig that was very nice. Gorgeous even.

I wish I still had it as I now have a 1911 again

Horizontal carry works well in a car such as the Galco Miami Classic or Jackass rig.  The size of the seat side bolsters can make a big difference i.e. sports seats vs.a plush Cadillac style.  Just tightening your belt doesn't work for everyone as it puts more pressure on the spine; wearing suspenders has worked well for a couple of people I know with bad backs.

I have a horizontal shoulder rig for my 2.5" K frames. I'll use it for woods walking or rarely if I'm spending a lot of time in the car and little time outside of it. Other than that I'm not a fan of them. It may be a Desantis - can't remember, but in any case it hasn't been made in a good while.

That does look nice. Vertical carry, attaches on both sides (don't want it flopping) and a double mag pouch.

Maybe I can even get them to make one that will hold a P226 AND P229.

I've got a shoulder rig that I love!  Forget if it's a galco or a G&G but it's leather and was about $125 4 years ago.  Super comfy andcarries two spares under my strong side armpit.  

Problem is it adds like 1.8" on either side of my torso which makes me look huge and requires something like a bomber jacket to conceal.  I don't wear big baggy stuff so it never sees use exceptaroynd the house or on road trips.

Lovejust watching Netflix with nothing on but my boxers and my shoulder rig lol.

Wish I would have gone this route in the beginning.  Their belt attachment method for the gun half of the holster looks much more secure.

I "cheaped out" and went with the Galco VHS and even with the optional tie down find the gun part too floppy.