After digging close to 30 holes in mostly red clay I have some observations.
1st. While it will go through some rocks, it won't dislodge those that are straight under it. I had to remove those to continue. The bit bogs down and stops.
2nd. Roots are another make or break issue. Small ones don't seem to give it much of an issue. Larger ones may or may not stop the drill depending on where the root is. Straight underneath, not much luck, on the sides, it can cut through them.
3rd. Be careful while digging in wet/muddy soil. If you don't bring the bit back up and remove the soil that collects on top of the bit a couple of times, you run the risk of the soil creating a vacuum and locking the bit in the hole. It wont turn then and with no reverse, you either have to man-handle it out of the hole (2-person job, trust me), or you are going to have to dig it out.
It does have some serious torque, so be careful where you drill, it can easily wrench itself out of your hands in a second.

So I have to say, this thing was pretty amazing. I had ten, 24" post holes to dig, and five of those were almost right up against an existing block wall so I knew there would be buried cement and God knows what else. I wanted electric, because my neighbors are pretty close and I didn't want to disturb them excessively with a gas engine. I had my doubts though, because I was digging Arizona soil, and I use that term loosely because it's just basically tan colored cement with all of the caliche. This thing made short work of seven of them, but three were real bears and I had to put my knee on it and really push but in the end I got them done and it didn't faze the tool. There's absolutely no way I could have done this job with a conventional post hole digger. Happy camper, well worth the money.

I bought 2. Does not handle roots well, but if you serrate the initial edges and paint with a good primer I think would do better. I plan to try this. I need deeper holes so I am going to weld my two together, then weld the inital cutter blades at an angle on the bottom with some cut in serrations. Rocks, as others have said, will stop it. For occasional use, soft clay / sand or small rocks (~1" diameter or less) soil, or if you feel like modifying, great deal, would buy again. Note a 1/2" conduit will fit all the way into the shaft, so you can modify it that way if needed.

Where I live, there is little soft soil when digging and using post hole diggers will almost knock your fillings loose! Plus, due to the rocks, a hole 8" wide by any depth takes at least 15 minutes to an hour. I needed something to dig a hole deep enough for a 20' light pole so it needed to get down to at least 36" deep. I couldn't justify buying a gas or electric auger for $300-$400 and have to get help to use it. I purchased this auger after reading the reviews and seeing that I could use it with a 1/2" drill (with a separately purchased adapter for about 10 bucks).
I started out drilling through about 10" of chert and kept getting hung up and having the drill stall. Rather than destroy a good cordless drill, I went with a 1/2" heavy duty drill. I was able to get my entire hole dug in less than 10 minutes and finished off the depth I needed with post hole diggers (only about 6" in softer soil). I highly recommend this for anyone who needs to dig post holes and not wanting to spend a fortune on a one or two man auger PLUS the auger bit. It does come with a few warnings from my own experience as follows -
**WORDS OF WARNINGS FOR DIGGING SOIL**:
*If you are using a cordless drill, make sure to use at least a 20 volt or better.
*If you are using an electric drill, make sure to NEVER put it in continual mode! If you do and hit something hard that causes the bit to stop turning, the drill will continue to run, taking a risk of personal injury to yourself or your equipment!
*The bottom blades are very sharp so keep your hands and feet away at all times!
*Make sure to let the drill brace itself against your left leg when drilling otherwise you take a risk with it getting away from you and causing injury to yourself or your drill!
*I found that using short bursts from the drill worked best for me until I got into deeper soil. It kept the auger bit from getting hung on roots or rocks.
*Make sure your drill has plenty of torque to be able to handle what this auger requires otherwise, you will purchasing a new drill!
*Finally, make sure you are physically able to use this with a drill. It should not be used if you cannot put a little muscle behind and it will give you a good workout!

I don't write reviews butI bought this to install a fence. 11/2021 it did great. Loosend the soil up, did not throw me around, 6'2 150 pounds. figured an electric one was more reliable than a gas one. I plan on using it again in the spring to install a 25 ft flag pole. Glad to have this tool in my collection