I started out renting a John Deere 450G dozer on my land and quickly realized it was too small for what I wanted to do. It couldn't break through the hard packed, dry red clay, and when pushing a full blade of dirt, it would quickly become too much and spin its tracks or kill the engine. It was great at shaping and spreading loose dirt. I bought a bigger Case dozer similar in size to a D6 at 170 hp. It does everything I want a dozer to do. It's also a never ending project of fixing things. I could never afford to rent one to do everything I want to do, or hire it out, so wrenching on it is how I save money. Hoses go all the time, cylinders fail one at a time until you've rebuilt every one of them. Then I think they start the process all over again. Branches are your worse enemy. Never run over them, they will catch on the tracks and then get through your belly pans and break something. I had a small, green pine branch bust off the oil sensor on the side of my engine block that created quite the mess of oil spraying through the engine covers!!!
Buying means that you have it there 24/7 and you can use it for as long as you want, or just let it sit until it's needed. Never expect it to work right away, there will always be something to fix on it. Renting means killing yourself to get the most out of it while you have it. Usually it will run perfectly while you have it, but hoses break, things happen, so even renting brand new, don't be surprised if you break down.
As to your original question, I personally hated my box blade. I never became proficient at it and found it to be extremely frustrating to smooth anything out with it. I tried digging ditches along my trails with it, but I just created a mess. I tried using the back blade for filling trenches when running water lines, but found it painful twisting in my seat going backwards all the time. After sitting around in my yard for ten years, I gave it to a friend who is now experiencing the same issues I had with it.....
Whatever you do, NEVER GET STUCK in the mud with a dozer!!!!!
Hope this helps,
Eddie