The van seats were 28 years old, and starting to be uncomfortable - no support, and the covers were getting thin, so I bought some cheapie sets on Ebay while I figured out what to do about them in the long term.
The seats themselves aren't bad, but they're not great. They're more comfortable, I think, but I'm pretty sure they're not going to last anything like as long as the originals. The thing is they come with no instructions, so I spent half a day figuring out all the wrong ways to mount the sliders.
First, put them on a soft surface, like carpet, so you don't wear a hole in the headrest while you're fighting with everything. There should be 4 holes drilled and tapped - check both seats before you do anything, as I mounted one seat, then found that the second seat only had 3 holes, requiring a replacement be sent out.
Arrange the sliders so they can be mounted - the larger side goes against the seat, but they still need the spacers or the seat won't slide. If you slide the thing up a bit, put the bottom bolt through, then slide it down and put the top bolt through, you can line it up so you can get the allen key through the holes in the bottom part when you mount it to the seat.
If you take the springs out of the sliders so you can put the bolts through easily, put them back in before you mount the sliders. There isn't the space to put the springs back in once the sliders are mounted.
Once the sliders are mounted, you can put the bar in to allow you to slide the seat. Be careful to pinch yourself between the slider and the bar as much as possible.
Now you can slide the mounts all the way up, and put the bolts through, and add an M8 nut to hold them in place, and all the way down to put the bolts through.
I ground out the sliders on the swivel seat mount, then drilled holes for the bolts. I ended up making the holes damn big, as you need to drill through the seam on the seat base, and I did a poor job of holding the drill in place - a large pillar drill would work better here :o)
Once I'd ground out the sliders, I put masking tape over the flat spaces, then put the seat on and marked where it should sit. In hindsight, it would probably have been better to do this on a piece of cardboard, and then use that to show where the holes needed to be drilled.
Once you've put the seat mount back in the van put the seat on it (I used 2 washers under each of the nuts to get it high enough to clear the VW sliders), and turn the seat so you can put a washer and nut under each corner to secure the seat.