I have a basic one. A Lienbach. It mostly sits. In fact, I should just sell it.
Mine does not have down pressure.
Unless the clay here is real wet, all this thing does is sit and spin on the surface. And, the few times it does bite, I usually hit rocks.
A neighbor down the road got a similar model, that has an extra hydraulic cylinder for down pressure. It can lift his 20HP Massey rear tires off the ground. It works much much better, but is also one of those $1200 expensive ones.
The best thing we have bought for doing all this kinda of stuff, is a Makita jack hammer. I think ours is a 45lb. We have a spike bit, and a spade bit. That is what we use. $650, but we use it all over the property. We have a generator; I just toss both in the bucket on the tractor. The spade bit works great for general digging, holes for posts, holes for trees ect.
Otherwise, rent a skidsteer with an auger for a day to do your holes. The auger that goes on a skidsteer, or the one that mounts on a regular tractor bucket, is significantly better than a 3 point version. More control, and you have down pressure. And with a skidsteer, usually more hydrualic power than your PTO. Only thing is, if you buy one of the big augers for the front of a tractor, or a skidsteer version, they are REALLY expensive.
In our area, there are some skidsteer guys that charge $4 or $5 a hole to come out and dig for you. They have a minimum charge of course. But if you have a lot of holes, it is worth it!
That jack hammer though; the best tool I have ever bought besides my tractor, for use out on the property. Best...!!!
Edit: I just looked; the price has gone up on these. But we have got every penny out of ours. A friend who does concrete uses his 20 year old one hard, every day.
http://www.makitafactoryoutlet.com/\makita\products\HM1500B.asp
I have seen them on Craigslist for half the price of new...