I'm a crisp single stage or long DA trigger kind of guy; I find that I tend to 'stage' two-stage triggers to just over reset under duress during a string of fire, and that's a bad habit. Sure, I'd like to get rid of the habit, but after 30+ years of trying I've decided that it's simply not gonna happen and so I've just adjusted to it by changing the types of triggers that I use. As a result, I've used USGI style, Timney, CMC, and
Jard single-stage triggers in my ARs.
Both the Timney and CMC trigger packs need (and supply) some form of artifice to hold the trigger in place (anti-walk pins, set screws, etc) due to the loss of the traditional AR FCG springs to hold the pins in location; the Jard doesn't need any help in that regard since it's effectively an adjustable AR-15 FCG.
I have used both Timney and CMC with straight and curved trigger faces; all of my Jards are old enough that they pre-date the availability of a flat trigger. I find that I prefer the flat trigger for bench work but prefer the curved trigger for field work.
My favorite trigger of the three is the Jard - I can adjust it for pull weight, sear travel, and overtravel whereas the Timney and CMC trigger packs are non-adjustable. I currently run with a 5lb return spring and a heavy/XP hammer spring Jard for my hunting rifles, and a 2 1/2 lb return spring /standard hammer spring for my bench guns.
I should point out that the springs in the Timney aren't as high-quality as I would otherwise like, but I've not had any issues with them per se. The need to deflect the hammer spring legs to install/remove the set screws will eventually deform the spring wire if you do this a few times, and IMO spring wire shouldn't do that.