You should for sure finish the show. However, watching it in real time through it's run and the drama the creative team put out about how it would wrap was disappointing. The fandom figured it out years ahead but they insisted it was something else, it wasn't. I owned all 5 seasons. I did not buy season 6 and sold the 5 I had. It's one sic-fi show I've fully divested myself of and don't care to ever rewatch.
I'm sorry but you are incorrect on this. The writers added the "fan speculation" part because of the speculation and it wasn't what people were actually thinking it was. (I'm sure that's a confusing sentence.)
Season 6 is a lot of fun and answers all the important questions while providing enough background to allow you to speculate on the rest. It also pulls a couple of twists that you just don't see coming. Finally the sense of closure to the series was very satisfying for me.
Fans criticisms tend to be similar to the ones people have about nuBSG. By the time you finish season 5, you know it's a fantasy show so just don't expect any "real world" explanations for the events.
If you guys are talking about them being dead the whole time, then no they weren't. The flashsideways were actually flash forwards to the characters all coming together again in the afterlife, years or decades later as they all died individually. At least that's the way I understand it.