I thought Sonic Forces was.. OK. It was really average.
Gameplay: My biggest problem was how loose the control was, and precise platforming was difficult because of that. Also, your acceleration is ridiculous. You go from 0 to 100 in a second which I found really irritating, and the extremely short & basic level design certainly didn't help anything. It honestly saddens me too because I love the aesthetics of the levels, especially Metropolis, but since the levels are so short & bland it's hard for me to get attached to them. Plus, when it comes to automation, this is one of the worst games in the series.
Story: I like the idea of the story, but ultimately I found it extremely unpolished. For a story based around war, the war aspect is ridiculously underplayed. We don't really see how the world or even much of its people were effected by Eggman's takeover. We also don't see the characters do much of anything beyond just talking. Classic Sonic was also completely unnecessary. It really feels like he was slapped in at the last minute.
Infinite I feel was a wasted character. I like the idea of Infinite being the leader of a mercenary group rather than just being a randon monster that Eggman found. Unfortunately though, his backstory is laughably bad. The scene where he decides to go mad was like watching a whiny immature teenager. However, I do feel that he has a presence, and part of that I feel is because of his amazing voice lol
But honestly though, it really seemed like the writers had no idea what to do with him. At least the voice acting & graphics were great. The jokes I thought were actually really funny.