I've used them and have had to replace three of them. One on a 270, one on a 300 weatherby, and one on a muzzle loader. What happened in each case is the point of impact would shift when you adjusted power. I don't use them any more. My 3200's and 4200's have been fine so far so I reccomend spending a bit more.
If you are looking for something in the lower ranges buy a good used Leupold.