I have bought slabs III and IV, and I really like the retro but fresh feel of Rob 'n' Bob's Slab VI. I have held off on buying VI because with the other two slabs and all the other patterned paper I have, I would have waaayyyy too much to ever use if I got another Slab at the moment. I would rather enjoy what I have instead of stress about the money and storage space, not to mention trying to use more patterned paper so I don't feel bad about the purchase. Also, I would much rather have the second pack of solid coordinating cardstock ProvoCraft has released.
If you can't get over the Slab VI, maybe consider picking up some Rob 'n' Bob patterned paper individually, or some other paper with that same cool retro feel that won't add so much bulk to your supply. Based on my experience with the Slab III, I can guarantee you if you pick up every slab that catches your eye, you will be stuck with hundreds of sheets of maybe a little dated paper patterns that were never quite your style to begin with but you had to have at the time, kwim?
hth!