I've had one for a number of years and I love it. I generally use the auto setting since, if you adjust it to ambient light at 7 a.m., the light when you're about to shoot at 10 a.m. will be different.
The only time I ever had an issue is when I was in fairly bright light and tried to shoot into deep shadow. I had to look hard to be sure of my target. That was one time of many uses.
The thing I find with a red dot is that, if your eyes aren't great, you may get some burst around the dot. It's generally NOT the dot. It's your eyes. If I put on glasses, the dot rounds right up with no star burst. It's still not a problem aiming, just a small annoyance...