Honestly a pretty good content drop for a free update, but doesn't fix my issues mentioned in my review of the base game and I ended up dropping it after the second trial. New characters are a cool addition, even if having to re-unlock basic skills was annoying. Tails was definitely the most fun out of the three. Not sure why it was deemed necessary to make knuckles pause in the air for a full second before gliding.
There seems to be a lot of complaints about the difficulty of the tower climbing levels. I actually appreciate that Sonic Team designed a really challenging platforming segment, but I think the problem is that it reveals how much Sonic in this game is not tuned for this kind of precision, especially since his speed is so reliant on boosting and air-dashing which results in him either not having the momentum to reach distant platforms or blasting off into oblivion at mach speed.
I quit during the second trial because I couldn't remember the combat controls and wasn't enjoying myself enough to justify sticking with it. Watched the rest of the campaign on youtube and I dont think playing the rest would have changed my mind. Boss gauntlet? No thanks! Final boss looked neat! Some of those cutscenes would have made 10-year old me's head explode!
Overall probably a great expansion if you like and are good at Sonic Frontiers. Just couldn't hold my interest.

Man, really wish I could get what others, fans and not, seem to be getting out of this game, but I just do not like it at all. There are a few charming aspects and steps in the right direction for the series but as a game its an unpolished mess of jank and bad design choices. Im not against this game's "open zone" idea, but turning sonic's typically stylish and creative levels into these barren landmasses that are as boring to explore as they are to look at makes for a bleak experience. I'm all for a collect-a-thon but grinding through identical drag-and-dropped grey rails and hula hoops gets old quick. Sonic controls okay until you realize this game has no momentum or weight to its movement. Requiring me to hold boost to maintain top speed feels bad on its own and is made worse due to the gauge being replaced with a BOTW-style stamina meter so I can't refill it on the go (unless I max-out my ring count which is so easy to do it makes you wonder why they bothered limiting your boost at all). Jumping out of a boost kills your speed and certain actions make Sonic stop on a dime so aggressively that it actually made me nauseous to play. The linear cyber-space levels would be the saving grace for me, except that these levels re-tune the controls to be horribly stiff.
Its not all bad- the bosses, while not exactly polished, are awesome setpieces. The story is refreshingly sober and actually takes its characters and lore seriously. Seeing a game where the writers and animators clearly think sonic is the coolest shit makes me feel like a little kid again, it's great! Unfortunately this just makes it more of a shame that this game, which is one of the more ambitious sonic stories in a while, has the presentation value of a fan-game and is one of the ugliest things I've played in recent memory. Still, the Metal Gear Rising influence is very welcome here.
Overall I get why people are enthusiastic about this game but it just doesn't do it for me. I play Sonic games for fun, stylish platforming and movement and levels and this game didn't provide me with any of that. Maybe next time...