The build quality seems phenomenal. It's heavier and feels very sturdy. You would struggle to break this thing. It includes standard sticks, two taller standard sticks, and two taller domed sticks, the two different d-pads, and 4 interchangeable paddles.The hair trigger switches for RT / LT are nice. Take them from a full pull to half pull, which for FPS games with semi-auto weapons makes aiming/firing potentially a little faster. For a game like Forza where the further you pull the trigger the faster you go, you can just swap it off. You could also go to half pull, but make it respond the same as full pull through the software - but then you'd have to use the software to correct it when swapping games.
The interchangeable sticks and D-pad pieces are also very nice. They are held on magnetically, but very well. There's no extra give or wobbling. I've been experimenting a bit with using a taller right stick, which in theory means it takes a little more movement on my part, which should in turn mean I can make more precise fine movements (FPS aiming...). I have not tried the domed sticks as I know I prefer the standard concave Xbox stick to the domed ones on the PS controllers.
I went with the 4 way D-pad, but can see where the circular one may be desired in some games.
Lastly, there are the paddles - these have the biggest possible impact to your gaming as you can map them to any digital button. Biggest use case would be mapping A/B/X/Y to them, so you could use both the left/right analog sticks and press these buttons without ever taking your thumb off the stick. However, these also have the biggest learning curve. The rest of the changes just work, but to use paddles requires potentially altering how you grip the controller, getting your muscle memory to press a paddle rather than quickly move a thumb over to A and back to the stick, etc. It will take many hours of play to get used to these if a person really wants something potentially game changing for themselves.
As mentioned you can map and alter things in software. You can even alter the sensitivity of the sticks, so for example make the first 50% of the possible movement less sensitive, and the last 50% normal, so for precision aiming if you stay in that first 50% things will move more slowly, but when you really need to turn around just hit the stick all the way to one side and turn at normal speed. You can have unlimited configurations created, but you can only store 2 configurations on the controller. There is a switch to change immediately between whichever two configurations you choose to store on the controller - so you could have a config for say a driving mode and for exploration mode for a single game and just flip the switch back and forth depending which in game activity, or you could have a config for FPS games and another for driving games and swap depending what game you were playing. Possibilities are pretty limitless with this thing.