I bought an 8.25" set of the hollow axle / hollow inverted kingpin trucks, and I love them. Great turn, great grind, great manual, great compliments from interested folks at my local park, great vibes all around. And I do like seeing that Slappy logo on the baseplate when I'm looking at my setup. I'm thinking about getting a set of the solid axle / inverted kingpin Slappys for my cruiser board.
I swapped out the bushings (the ones I like are much harder than the stock bushings), and then I put a pair of hollow Krux DLKs on them too, and they're spectacular. The kingpin swap may be more for mental reasons than anything else: Those Krux DLKs weigh 20 grams, the hollow inverted kingpins that come with the Slappy's are 25 grams, and the solid inverted kingpins that NHS makes and puts in their Indy and Krux inverted kingpin trucks are 30 grams.
I was riding a pair of Frankenstein'd Independent Mids with a hollow 144 axle and the same kingpin and bushing setup as above, but wanted to give Mike's trucks a try.
I am worried about how the axle nut just sits in the Slappy Baseplate (it's a hex bed setup) because I had a set of Krux K4's that were built like that, and over time the axle nut stretched out the baseplate and those kingpins would move around laterally. If that happens with the Slappy Trucks I'll try JB Welding that sumbitch in place, or if the baseplate gets fucked, try putting the Slappy Hanger on the Indy Mid Baseplate (I do like how the kingpin screws into those).