I'm pretty disappointed.
Some negative things:
First of all I hate being timed. Health doesn't matter since you run out of time quicker. I think in arena your primary goal should be survival and round should end when you die.
Secondly, double jump is gone. I feel like nimble agility present in previous games is replaced by a floaty moon jump. Landing on platforms is so much harder now.
Thirdly, infinite ammo strips all challenge from the enemies. Might as well have no foes and just wander around and find items and have a similar experience.
Finally, not to be shallow, but no medals - no real incentive to play.
Now the positive:
Customization was improved from the previous game. I loved the freedom to choose colors and actually see all the variations instead of cycling through one-by-one.
Overall, I guess I was too spoiled by TTA3. That game was pure brilliance, totally fun and gave complete freedom in terms of game mode customization and in ways of dispatching enemies and in traversing through the arena.
This game, on the other hand, feels far too linear, involves a lot of trial-and-error to figure out where all the discs/parts/etc are, all while mindlessly holding down the left button to endlessly spew bullets unto squishy walking potato-bags enemies, and wrestling with sub-par platforming mechanics.
Sorry if I'm being harsh, but that's only because I know you can do better.