I have them. Straightforward is a good word, they’reeasy to bleed, easy to set up, easy to get along with. There’s heaps of power, good modulation, and all at a low price. Really like the feel, dead happy so far. Starbike.com were cheapest when I bought mine.
Also have Cura 2s which are good too, but more XT-like in their immediate bite, that’s the main difference to the 4s as opposed to power, they’re easily the most powerful 2 pot brakes I’ve used.
I was wary of Formula having had less than positive experience of their older models, but these are a different beast. The reach adjust is annoying and needs a small Allen key, that’s my only gripe.
Very detailed reviews of them on nsmb.com, worth a read.