I installed this last night and played the first hour or so. First impressions, it's more of the same, but with an awesome THPS-inspired punk/rock soundtrack. If you've played Trials, HD or Fusion, you know what to expect here. The layout is very weird though, as @mfnick alluded to. The world map seems like a strange decision, rather than the linear progress you made in the other games.
Even something like Horizons (the phone game) would have been preferable to this.
The lootbox side doesn't seem to insidious on the grand scale of things to me. I haven't seen anything I NEEEEEEED that might persuade me to purchase more lootboxes. Right now it just FEELS like something to make you feel like you're progressing, rather than just something to shill cash out of you. I've also had no issues with unlocking bikes - just managed to get my second one, but I feel like that was just a levelling up bonus.
There's a couple of great new ideas there too though. You get to a point where you're offered sponsored races, and eventually sponsored mini-tournaments. This is where the multiple opponents comes into things. You never HAVE to beat opponents to get medals, until you get to these little round robin style moments, where eventually coming first is a must. They're a nice twist on the singular Trials experience.
I also really like the training stuff. There are about 15 or so lessons, all unlocked as you progress through the game, each one teaching you a different skill set. The last one I did taught me effective jumping, and was a really good lesson. Not only did it break it down visually, but then it asked me to have a go, trying to get all the way up to an A+ rank, and also had an opponent doing the same thing so I could see how to perform the manoeuvre in real time. Great way to do it.
It feels like a game I'll drop in and out of until it's really got its hooks in, but for now it's just good, chilled fun.