I bought my first Glock, a 17 in 1987. I still have that gun, having carried it as a Border Patrol Agent when the patrol still allowed personally owned handguns. It was always reliable and held up to a lot of abuse. Its been packed full of dust, banged against rocks, and dunked in the Pacific Ocean once or twice. Its had a lot of rounds thru it and probably half were +P+. Even with a lot of rounds thru it, it was the gun I still chose to fire for record when I was selected to attend an FBI firearms instructor school. I currently own five Glocks in 9mm, 40, and 45.
I saw my first Sigma when they were first introduced in 9mm and recall that I noted the similarity between them and Glocks. Later I heard that Glock had sued S&W for infringements, but I never heard what the official outcome was for that. My personal experience with the Sigma is limited to a few rounds here and there. About the time that the Sigma was acquiring a bad reputation, S&W's ownership at the time was getting in bed with the Clinton regime, which didn't go over very well with honest American gun owners.
A police department in my area, like many, were die-hard S&W throughout the transition to semi autos. They authorized a 3rd generation pistol for years with good results until S&W discontinued that model. Then they switched to Sigmas in 40. The Sigma experience was apparently not what they expected and they switched to Glock. I still encounter their officers who are grandfathered to carry the Sigmas. Some carry guns that work. I've also seen some officers carrying Sigmas that needed some attention from an armorer, to say the least.
On the range, I am very surprised to see a Glock fail to function reliably. Still, they are mechanical devices and like any mechanical device, they can fail. I've also seen Sigs fail to function reliably when springs and other parts start to wear out or break. A Glock is down-right simple to fix in comparison with other pistols.
I recently purchased and M&P 45 and am thusfar very impressed. In my opinion, it is a better gun than the Sigma, and has some features I like over a Glock. The M&P will rightly give Glock some worthy competetion. I grew up on S&W revolvers and glad to see Smith producing a semi auto that is rock-solid quality and American made.