I realize this thread is a year or so old now, but for future reference here's my $0.02 on the Shaver post drivers.
I have two shaver HD-10 drivers, one hydraulic tilt skid steer mount, another 3pt hitch, manual tilt. About 25% of my agricultural construction business is fencing, so I don't run them every day, or even every month, but I probably do drive 1000 posts a year. A few things to keep in mind, regardless if it's an HD-8 or HD-10:
The newer models that have the foot on them are far superior to the older ones that were suspended on the 3pt hitch only. On the 3pt model you do get some side to side sway with only the foot down, but I haven't found that to be a problem. While driving you can put one hand on the big hyd hose on the cylinder as a handle and keep it plumb and stable if need be. Having the foot on the ground takes almost all the shock load off the tractor and makes the driver hit harder too.
Springs should last many hundreds of posts, if not something is wrong. I only change springs every few years using mine semi-commercially, just a WAG but probably 2000+ posts per set of springs.
Unless the ground conditions are just terrible you can absolutely drive a straight, true, plumb fence with them, even the 3pt hitch manual tilt models. The learning curve is steep though, there are lots of little tricks to make it do what you want. The hydraulic tilt models do let you muscle the posts in line better, but everything is doable with a manual tilt, in a nut shell:
-don't point the posts
-take a spade and dig out a small starter hole 4-6" deep if alignment is important
-plumb the driver side to side, set it down, and it should be close to plumb front to back.
-start driving the post with it leaning in towards the tractor a little. After it's 12-18" in the ground, push back with the tractor a little to bring it plumb.
-try to keep the post plumb side to side as best as possible. After a few hits check it with a level, push it over by hand a little(NEVER put your hand on top of a post, only grab the side), then drive some more. You can also angle the driver so it's hitting the top of the post a little at an angle to put some english on it. Every hit will drive the post over a little opposite the direction you have the driver leaned. In really tough cases you get the post driving plumb front to back, then move the tractor 90* so you can push back with the tractor while driving to keep it straight.
-the goal is to get the post going straight within the first 18" or so of driving, after about the 24" mark they're pretty much set on the course they are going
-if after driving you have some that are out of alignment, take the loader bucket or frame of the post driver and push the post straight. Push a little past straight, then tamp around the post to keep it there. If it still wants to spring back cut a wedge out of an extra fence post and pound it in behind the post to keep it straight. Experience will dictate how hard you can push on a post before they break, a big 6-7" corner posts in soft ground you can push pretty hard on, a little 4" line post in hard ground will snap if you try to go too far.
Hope some of this helps. Years ago I thought the Shaver drivers were impossible to drive a straight post with, because every fence I saw built with them was a disaster. Decided to give one a try myself, and after a steep learning curve I figured out it wasn't the driver but the operator that was the issue. Here's a board fence for a riding arena I built, driven with a HD-10 manual tilt on the back of my little Kubota tractor: