This is a review I should have done a while ago as I've been using ALS (Automatic Locking System) holsters for a while now. Safariland didn't actually send me anything so I didn't have the usual feeling of obligation to get something up in a timely manner, however I tend to always think better late than never. In summary when it comes to a full on combat / duty style holster, I think the ALS series is the best thing out there.
Now what I expect out of a combat holster is: durability, some form of retention, and an easy release method (yes jokers, we'll assume "safe" is a standard requirement as well). Durability means it can survive combat abuse of being banged up all day and preventing an enemy tearing the weapon off, retention beyond just friction so you don't ever have to worry about your weapon falling out, and an easy and reliable way to release the retention so it does not take too many steps or leave one's hand in a non usable grip, the ALS series fits these key points quite well.
Not that long ago, the ol' Safariland 6004 was one of the go to duty holsters making a great balance of tactical expectations at the time, but it did leave room for improvement. It was a hassle to always have to flip the hood back in place to fully secure a pistol after re-holstering and the thumb movement required to release the hood felt like a 2 part process to do consistently. The ALS system smashed these issues where now the retention is spring loaded grabbing onto the the ejection port rather than the back of the gun so the weapon is automatically locked just by placing it into the holster. The placement and rotation of the release lever is also genius where it is naturally activated when going for a strong combat grip. One doesn't have to push some random thingy in some other direction, the thumb just swipes it naturally.
While the ALS design had been out a while, cool guys started to put optics on their pistols and Safariland followed up making the secret squirrel Model 6354DO ALS Optic Tactical Holster. This thing wasn't even on their website for quite a bit of time and had to be special ordered by those in the know. Originally intended for Glocks with a Docter Optic, as the name implies, the ALS mechanism had to be reworked since where it originally mounts in the upper center, is space where the optic needs to be. As a result on the DO variant the ALS system connects at the sides with a separate spring for retention rather than straight material tension as seen on the original. Apparently the pistols that the 6354DO model was made for either had rear iron sights in front of the optic or none at all as I've had to make some mods to make my G17 with RMR and suppressor sights fit. I've got mine with co-witnessed suppressor height iron sights, the rear being behind the optic to not lose sight radius, the way Jesus would want it. So anyway, I had to shave the optic hood zone and the internal security piece down a bit (shown in the pics) with a Dremel to get the extra space I needed. Fortunately it wasn't anything too dramatic and the internal area I shaved down is no where near the important ejection port locking zone.