I've owned or been issued eight Sigmas. I used to shoot Glocks, but after being issued a Sigma, then picking up a few on my own, I ended up trading all five of my Glocks away. They just don't feel right in my hands. And, I shoot cast bullet reloads, so the Glocks isn't the best choice.
First off, here are some of the "problems" reported with the early Sigmas. I have only experienced one of them, and that was the cracking frame. EVERY early Sigma I've owned has cracked on me. But, S&W stands behind their products and replaces it with a currently produced Enhanced Sigma under warranty.
1st and 2nd Generation Sigmas were known for cracking frames in front of the SN plate on the bottom of the dust cover. They start out small, then get larger, but never effect the functioning of the pistol. The addition of the equipment rail made the frame more rigid, and there has not been a single Enhanced Sigma returned to S&W for a cracked frame. Problem solved.
There were many broken strikers in the early 1st/2nd Gen Sigmas. That's because they used the Glock-style loose tolerance chisel tip strikers. I'm assuming it's due to the recoil and vibration which causes the breakages. I don't know, ask Glock since their G19's have been doing this at an alarming rate with the Glucks in service in the NYPD. To the point of NYPD threatening Glock with the cancellation of their contract (with one being awarded to Ruger instead) unless they worked at fixing the problem. Right now, there are three full-time armorers working at the NYPD firing range attempting to correct this problem. S&W solved the problem with the Enhanced Series by redesigning the entire system with a round striker, with all the parts machined within .002". Again, no more broken strikers reported.
Some ejection problems were reported. The ejection port was lowered and reshaped. Again, problem solved.
They did some cosmetic changes, which included moulding checkering to the sides of the grip portion of the slide and rounding the top edges of the slide. They also added a loaded chamber indicator, which amounts to a hole in the slide/barrel so you can see the rim of the chambered cartridge.
I own, have owned, or been issued these Sigmas: SW357V, SW40F, 2-SW9F's, SW40V, SW40VE, SW40G, SW9VE and SW9GVE. I have shot tens of thousands of rounds out of them, and I have had one, yes, only ONE malfunction. I had one FTE in my issued 1st Generation SW40F. But, at that time, I had over 8000 rounds of mostly cast bullet reloads with Unique powder through it. I'd have to say that one in >8000 rounds is an acceptable number. In all of the other Sigmas, both older and Enhanced, I have never, EVER had a malfunction of any kind.
Some people dislike the Sigma's trigger and say it's too heavy. It's right around 12 pounds, the same as a Sig DAO. For some reason, it's deemed excellent in the Sig (which I don't agree with at all, especially since the Sig has a much longer trigger pull), but people don't like it in the Sigma since you pay less than $300 for it.
There is a trigger mod detailed on the S&W Forum. I strongly advise against it, and my reasons are posted in that thread, several times. For a range ONLY toy, fine, but reliability may suffer. By doing this, you are reverse-engineering your Enhanced Sigma to an early 1st Generation gun.
After about 500 rounds/dry fires, you will find the trigger smoothes out considerably. If the pull is difficult for you, I have a simple way to train with it, and it has not failed anyone yet:
Just sit in your room, at the computer, watching TV, whatever, with your UNLOADED Sigma (had to throw that in there). Just practice a slow, consistant trigger press while keeping the sights aligned at a spot on the wall. Just keep on doing this, over and over and over again, for weeks. By concentrating on the sight picture and keeping it on target all the way through the trigger travel, you will find your hit ratio at the range improved dramatically. And, as an added bonus, your trigger will be smoother at the end of it.
The early Sigmas were not as bad as some people want you to think. In fact, I'll still pick them up when I find a good deal on them. But, the Enhanced Sigma is refined to the point that it's truely an improved Glock. It has cut rifling so cast bullets are completely safe (where shooting cast bullets in a Glock void the warranty, AND could cause the famed Glock KaBoom!). Sigmas have ample chamber support, so reloads and high pressure ammo is completely safe. No bulged cases like the Glock, and I've NEVER heard of a Sigma KaBoom! The grip angle is natural on the Sigma. It aligns naturally, unlike the Glock.
And, I don't know if anyone else has this problem, but with my big hands and a two thumbs forward grip, the left side of the slide always cuts my hand during recoil. I've always shot with a high grip, but I've not had this problem with any other pistols.
When it comes to the SW99/P99, it just doesn't fit me as well as the Sigma does. None of the interchangable backstraps fit me just right, and, although the trigger pull is lighter, it doesn't feel as good as the Sigmas. The SW/P99 is probably going to start to get phased out if the M&P catches on, but the Sigma is expected to last.
Do I feel the Sigma is the best pistol that S&W makes? No, I'd have to give that title to their 3rd Generation autos. But, for the price, I'll always have at least one Sigma in both 9mm and .40, but usually more than that.