As a standalone thing, I don't think it works at all. As a straight Tony Soprano origin, it doesn't really work that well either. It's pretty watchable but it's also a bit odd, not really about much and dips in and out of surrealism in a confusing way. Skipping around from one thing to another. 
The main players in the cast are all pretty good. The young versions of the rest of the crew are completely baffling. I thought that Pussy and Sil were the same age as Tony. There's definitely lines in the old series about them coming up together and being at school at the same time. But those guys are already in the crew while Tony is about 8 years old. The character playing Young Sil looks so old I thought he must be playing his Dad and spent most of the film thinking it was stupid that they had his Dad walking around with all of Sil's mannerisms. It was only at the end they called him Sil and I realised who it was. Junior shows up and looks so much like Artie Bucco I thought that was his Dad. The best bits are the sporadic acts of surprise murderous violence, which at least take the script from doing not much to doing something.
This might end up being the first of a few prequels they do with the same cast over time. I'd definitely watch more because it has its moments. But the fan service is clunky, and I dunno, Sopranos shouldn't be doing stupid origin and fan service stuff. Equally though, if they do it, I'll watch it. Low-key, period mafia drama. It's fine.