There is a topic that is taboo on most brewing discussion boards. Many people have heated arguments over it. In homebrewing, it can be more divisive than religion or politics. Regardless of all that, I'm going to weigh in, and give my opinion. That subject is grain mills.
Grain mills in brewing come basically in two forms: the Corona mills and two-roller mills. Many people have made fine beer with both.
Looking at the Corona mill first. It is the oldest grain mill used in homebrewing. It works by grinding the grain between two plates. They are primarily designed to to make flour for baking. Therein lies the problem.
The problem with the Corona mill is that, because it was made to make flour, it can mill the grain too much. This can lead to stuck sparges. One has to pay close attention to the adjustment to make sure this does not happen. They are priced right, however, at around $60.
The most popular mill in homebrewing, by any stretch of imagination, is the two roller mill. There are many brands: Valley Mill, Barley Crusher, JSP Malt Mill. All will give you a good crush. All are more expensive than the Corona mill. Price wise, they are two to three times as much. They work by crushing the grain between two knurled rollers, leaving the husk intact, but breaking open the contents -- perfect for brewing.
I happen to use the JSP Malt Mill:

"Whoa! Wait one cotton-pickin' minute! What about the Barley Crusher? I mean, it has all metal construction. And, the rollers are adjustable at both sides. And, by the way, the Corona mill makes grist just fine, if you are just careful. You and your stupid Malt Mill. I mean, sheesh."
Sorry, I am a JSP Malt Mill user, and I'm happy with it.
When I was in the market for grain mills, I weighed many options. In fact, the local brew shop used the Valley Mill. I priced them out, and shopped around. As it turned out, the homebrew shop in Flagstaff, Arizona, The Homebrewers Outpost, carried the JSP Malt Mill, and I was traveling to Flagstaff for an SCA event. I did a little research. People seemed happy with it. They were priced well; I think I paid about $150 for mine. So, when I went to Flagstaff, I bought one.
The JSP Malt Mill uses two knurled rollers fabricated from cold rolled steel. The spacing between the rollers is 0.045 inches, which is just narrow enough to crush the grain without turning it into flour, leaving the hull intact. Part of the housing is made of wood, as is the hopper, which holds about three pounds of grain. It comes with a hand crank, though there are places online that document how to motorize it. There is a wooden base that sits perfectly on a five gallon bucket.
To operate the thing, I found that if I brace the bucket between my legs, and push down on the wooden base with my left hand, it gives me enough stability to turn the crank with my right hand. Here is a picture from a few years ago demonstrating the operation. Notice how sexy I look in my socks:

I opted for the adjustable Malt Mill. However, it only has a adjustment on one side, though I have to admit that I've never adjusted it in all the years of usage.
After I bought the thing, I noticed a jump in my efficiency. It went from about 70% to about 80%.
I've put many pounds of grain through the thing and have never had a problem. I highly recommend it. If you are in the market for a grain mill, give the JSP Malt Mill your consideration.