The reviews I've read have all said it's very good and the 3rd act is superb. The main criticism seems to be that while the characters are good, some of them are under-developed - but I suppose that's to be expected from a one-off film that seems to be a kind of Dirty Dozen style film where it's not all about two or three main characters. The spin-off films are unlikely to have the same emotional resonance as the main saga ones because you don't get to follow the same characters through three films, and because they lack the Skywalker connection. But they can still be really good films.
I think we will see a bit more deviation from the formula than with the Marvel films, partly because they're using different writers/ directors for each one, but also because some of them will be set at completely different times and with completely different characters, so they won't have to maintain exactly the same style / tone. Rogue One apparently is more like a war film than TFA, and I think the Han Solo film will be a bit more comedic. Also I think Episode 8 is going to be a bit more 'out there' than TFA.