If it wasn't modular it never would have been adopted as a service pistol considering the US Army's bid competition was called the "Modular Handgun System", and maybe it would be far less popular with everyone else (maybe even abandoned by Sig?) as a result of not achieving that bid.
Anyway, I own one but I haven't shot it yet. Feels nice in the hand, but not a fan of polymer grip, stock trigger is OK. Points naturally. I've also shot an X5 Legion and the trigger was better and it weighs a bit more, so that was that.
I prefer my all metal hammer fired SIG's trigger and weight though. I bought the p320 for only one and a half reason: modular opens up options in our gun control country. And it's a Sig.