For the life of me, I can't figure why there are one-star reviews for this mirror. I installed mine in 15 minutes with no problems at all. The screws are very difficult to tighten, but the instructions explain that's normal. I did use a longer allen wrench from my toolbox instead of the included one so I could get more leverage to tighten the bolts. The included one would work, but it would take a bit longer. It was easy to adjust and has a wide field of view.

Can't believe I haven't written a review for this yet. I love this mirror, had it on my bike since Feb 2016 (now Jun '17, so 16ish months) and it still works great. It makes my handle bars a little wider but I have folded this down and around to fit in snug places quiet a bit and haven't had trouble re-adjusting it when I get back to peddling. I don't like mirrors that hook to the helmet and this is probably the best one for handlebars in existence.

March 2018 update: It broke, and I'm in Mexico, which sucks. So I have been going between cars at lights in the cities in Mexico and it has gotten knocked around a few times. I believe the first good hit was in Tampico. It broke but I think water corroded it and weakened it. The bolts also aren't stainless, if I bought one in the US again I would search for the bolts in stainless. I am buying another one though, I found an American here in Tuxtla who is having a sister fly in next week for a visit and she got one and is bringing it. It really is an amazing mirror and I highly recommend it. However, if you're touring buy a new one and keep the one you've had for a while as a spare. I'm entering Guatemala in a week of riding and I hear a mirror is a must there!

I have never had a mirror, but after installing this and using it I'm not sure if I could ride without one! It's so nice not having to turn my head all the time. The view is very clear and wide, it's easy to adjust and it stays put.

Installation was simple. I was a little skeptical about cutting the end of my grip, but after doing it I have no regrets at all. I used an X-Acto knife to get a nice clean cut following the edge of the handlebar. Any sharp knife, razor blade or even a sharp box cutter will do. Just be careful not to cut yourself! I found it very easy to install and actually fit very snug with the smaller insert. It comes with two handlebar inserts, both having the ability to adjust to various diameters. It essentially expands as you tighten the hex screw on the outside of the mirror. Others have mentioned that duct tape helps with any slipping; I have not had to use any yet, but I can easily see how it can help if it doesn't grip to the inside of your handle bars. They also sell spare mirror replacements in case you break this, which is nice. (It is a convex piece of glass, which is one reason why it's so clear.)

If you don't want to cut your handlebar grip, you may want to look at helmet mirrors. To me they seem too small, but I have heard some good things about them.

I bought two of these almost two months ago and I returned them because the small cap screws did not fit. They were so tight due to poor machining. I reordered them thinking I had gotten a bad batch, but I had the same issue with the replacement mirrors.
So, I googled the issue and sure enough I found many people had the same issue, and yet so many positive reviews for these made me determined to try to find a solution.
First, I went to a local bike shop and they did indeed have this product at $6 higher than the Amazon price. I told the shop owner my problem and he told me he sells gazillions of them and he never had any complaints. (Gazillions sold from a small bike shop?!)
So, home I went. I am a retired tradesman and have almost 40 years experience dealing with screws and such. I didn't have a tap and die set, so I went down to Lowe's with a cap screw and the plastic piece with the female brass thread in it figuring I could find a better matching screw.
First problem is the brass female insert and galvanized screws are a poor combination. For one, dissimilar metals touching cause galvanic action which is corrosion. I used the sizing templates at Lowe's and thought I found matches. The screws were not the same as the inserts! The screws are 8/32 but the insert seemed to be an odd size I couldn't match up! The closest was a metric size but it wasn't a good fit. So, home I went where I found a good solution.
I put one drop of 3-in-1 Oil on the tip of the screw and started to screw it in until it began to get tight. I then backed it out until it got loose and did it again. Again, I backed it out but all the way this time because the screw got hot. I wanted it to cool off because the heat would have made it expand in size. While it was cooling, I did the same with the other screw, but with the drop of oil on it I was able to insert it all the way in. I then tried the first screw after putting one more drop of oil on it. It went in all the way that time. Now, a note of caution. As you begin to tighten the screw, test the swivel motion of the mirror to make certain you don't over tighten because the plastic parts are hard plastic and they WILL crack if you over tighten. You want them just tight enough so they swivel with a little hand pressure but not on their own. Also, be just as careful when you tighten the long screw into the handle bar, tightening just until it seats the mirror without it moving.
I like the mirrors and I will purchase them again if I ever need to replace them. I gave three stars because the manufacturer has yet to correct the machining problem that they must be well aware of.
My bike is a Trek Shift 4 and my wife's bike is a Trek Pur. I used the small wedge to attach the mirrors to the bike. No issues installing them. Attached are a photo of each mirror installed for a better idea of what they will look like installed. You will have flexible options on the height of the mirrors according to the amount of angle you install them with.

This is the third Mirrcycle mirror I have purchased. I found all to install easily with the included tools. Even though designed for mountain/off road bikes, I have one of these on my road bike and it installed and works well. The pivot connections are tight so adjustments stay where you put them. The fold back design protects the mirror from damage if you travel through tight places.