I transitioned from an entire rack of dumbbells to the NÜOBELL 580s for two reasons. The first, and least important, was space savings. To be totally honest, that was an afterthought as I had used the rack of weights for 20+ years. It wasn't until after replacing them with the single set of dumbbells that I realized how nice it is.
The real reason for purchasing the NÜOBELL 580s was to address the gaps in my existing set. Folks that have priced them know how expensive it is to get an entire suite of dumbbells in five pound increments. For that reason, my existing set had large gaps of between 10 and 20 pounds. Problem solved. (for reference, my current set has 5, 10, 15, 20, 25, 35, 45, 55, 65, 85, 100)
There are other options on the market, of course. But those had too many shortcomings in my book. Things like goofy plate selection or limited total weight. Also, for certain exercises I rest the dumbbells on my thighs before lifting them into position (i.e. incline DB press). Other vendors' offerings weren't conducive to that.
The build quality has proven to be outstanding. When new, there was some stickiness when switching weight selections. However, turning the handle the opposite direction just slightly freed it up and selection was easy. Note that this issue seemed to be only during a brief break-in period.
My final thought -- more for NÜOBELL than for folks reading this (long) review -- is this: it would be nice if there was an option for 100lb dumbbells. The fact this option is not currently offered does not detract from the five star review as I got exactly what I expected with the 580s. However, perhaps a future expansion pack similar to the one that exists for the 550 to 580 conversion?