I finished playing Trials Fusion today (I know you cant) but well as far as singleplayer goes.
Tried out a few of the TrackCentral Tracks. So i'll start of with what I like about the new game.

I like the commentor, bringing in a little story into Fusion while playing is a nice idea. The Editor is still fine; few additions but it is already very complex and if you wanna go further it is questionable, if you wouldnt wanna go into programming altogether. Bugs are rather critical at this point but still neither HD, nor Evo were perfect at launch. So giving them time to iron out physic bugs should have been expected. FM System is Fun as long as it would only work for specific tracks. I think it doesnt belong into regular gameplay/speedrunning since it is so complex, in sense of physics happening, that getting a better time between the top riders boils down to luck more than skill.

So and here we are coming to the darker side to Fusion. Trials was a nice combo out of regular textures, a little muddy and sticked out by it's object colour themes.
But where Trials Fusion went with it's future setting; the hard, cold metal textures take away both overview and recognition of movement. They are also quite monotone and hardly fit together with the traditional scenery of Trials. Meaning that tracks based of wood and other objects have a hard time right now (although some track editors managed to do the trick). And then the absolute downer for me was the music, i mean hd and evo had such catchy music and there it was basically just bling bling bling, even the higher pitch of checkpoint sound should be lowered. Getting a rythm is hard. Also tracks arent really recognisable anymore. In Evo I would know every track by heart within a week and instantley recognise them. In this game you need far longer to know the Tracks showing off another big problem. The limited variety of the theme itself.

I would say the game is better than Trials 2 at the moment but still lacking far behind Evolution both in terms of music, variety (basic tracks) and overall features such as multiplayer. Also loading times are longer compared to Trials Evolution and the overall menu was more convenient to look at.
If you compare it to HD now, it is no easy call as it should be. HD had some tire bugs as well but still Track and Music variety. Also even though the tracks were all (!) set in one single Warehouse they had far more feel and joy in them than the tracks in Trials Fusion. So at the time I would say Trials Fusion ranks between Trials 2, which had very basic textures, good levels, but still repetitive design, Trials HD and even a little more behind Trials Evolution. Trials 2 was a very good game, but for today's standards as hard as it is Fusion is just average and slightly below average considering the quality of respective Trials predecessors.