Here's an opinion from someone who uses them quite often. Everyone so far is correct. You need to know the size of your target for the ranging to be accurate. Even then, it takes a lot of practice to get proficient with it. Don't expect to go out once or twice and be able to range within 10 yds of actual distance.
Also, the magnification that the reticle is calibrated for is extremely important. Some scopes are slightly off, so you should calibrate your setting using a yard stick at 100 yds.
All new scopes that I buy are now mildot or some variant. I use them for ranging targets when competing in sniper matches where the target is at an unknown distance and rangefinders are not permitted. Outside of that arena I do not use them for ranging. However, I use it quite frequently to hold for wind and follow-up shots. For instance, when I varmint hunt, I dial my elevation after using the rangefinder. Because wind shifts quite frequently, I typically use the mildot reticle to hold for wind. If I miss and see the impact, I can quickly adjust my hold using the mildot reticle and follow up with a second shot much sooner than making turret adjustments. This allows me to get a shot off potentially before the groundhog goes into his hole, as well as fire again before the wind conditions change.
Bottom line: the mildot reticle is not just for ranging. It can be a very helpful tool if you are proficient using it.
BTW, there are better options than the standard mildot. Leupold's TMR or Nightforce's NP-R2 are just a couple. They offer much more granularity, which increases their accuracy.
