I've never used them on anything that I built but I have owned a couple of boat trailers that had similar ones on them They were fine when they were new but over time they wore out and the tires had more camber than my old dirt car. It's hard to beat the durability of a solid axle with leaf springs and it looks like the price of both setups is about the same. jm2cw
I must add that I'm not sure what you're building but if it is being pulled up and down the road a lot then I'd probably stay away from these.