Some don't like it at all, but I think it has its place. The problem you run into most of the time isn't with the machine, in my opinion, but with the feeder. Inconsistent feeds, where the feeder fumbles around trying to find the hole to put the ball in, make it difficult for hitters to get their timing. Slow arms circles with machines set to 60-70 mph also throw off timing. No arm circle that slow would produce those results.
So if you're going to use it, the feeder(s) has to learn how to do it to give a more realistic look to the pitch. That can involve holding a ball in the chute with your left hand while the right arm makes the "circle" and release the ball into the chute at the right time. It can be a rhythmic show the ball and then stick it into the machine at the same time each time (no circle required). But something that gives a sense of this is when I need to be ready to swing.
Mostly, though, you'll want a steady diet of tee hitting, front toss and live BP if you can. The tee works on building mechanics, live toss works on applying them when there's a ball moving toward you. The machine (done properly) helps hitters learn to see the ball over a more realistic distance and get the feel for closing speed.
Your ultimate goal is to build a confident hitter - one who believes in her mechanics and can apply them with enthusiasm at the plate.
We didn't use a machine much, mostly when they were very young and our pitchers weren't consistent. As others have said, confidence booster. One thing the machine did help, and I have no scientific proof, was teaching how and where to lay the bunt. The machine really seemed to be an aid that transitioned over to live pitching.
It has its place. Unfortunately it is often over used and gets summarily dismissed as unproductive. You would be hard pressed to find a successful program that does not use machines, front toss, tee work...
Our team regularly uses a pitching machine. It is one of many hitting stations, like riseball suggested. Ken's point regarding the person feeding the machine is critical. Simulating the pitching motion gives the hitter time to load and not feel rushed.
I'm in the front toss camp. Very rarely do I use the machine. My biggest complaint is that the balls take an unrealistic path to the batter because of excessive the backspin.
I posted this before. I made it for a really Great sponsor, turned coach. He went out and bought his own pitching machine to put in his garage, to teach his DD's. I was showing him some machine settings...
We use it to hold the corner up in my shop. Looks new and 4 years old but only used 2 or 3 times a year. I find myself bringing it to practice if I know my pitchers are not there or needing a break. I have another set up in a cage that we do use but it auto feeds the balls to my daughter while daddy has other things to do.
We use 1 regularly as well. 1 cage pitching machine, 1 cage front toss, 1 cage line drive drill, and tee work into the bow net. We place the pitching machine close at 35' and have the speed around mid to low 50's.