These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
In Extreme-G 2, the Zeo-Max is glitched such that its maximum speed is freakishly high, outstripping unlockable bikes Wasp and Spectre.
Extreme-G 3's Railgun. Some may find it hard to use, but with properly lined up shots and full charge, it only takes a couple of hits - and a few seconds - to destroy another bike. Combine this with the ammo storage upgrade and a good player can destroy three or four bikes without refilling weapon energy, and that's only assuming each shot hits only one bike, because shots travel through the bikes they hit. Lastly, railgun shots do not need to hit exactly, at least in the PS2 version.
Goddamn Bats: The AI in Extreme-G 3 is incredibly fond of using mines, which become harder and harder to dodge over as the speed limit increases as you progress throughout the game. Getting hit once or twice is acceptable, but when every single bike ahead of you will drop dozens of mines...
Most Wonderful Sound: Definitely the sonic-boom you hear after breaking the sound barrier. You know you're going to rock the competition when you hear that.
Scrappy Weapon: The mortar cannons are difficult to hit things with, but annoyingly common.
The second game was a decent game in its own right, but is widely regarded as a step down from the (admittedly outstanding) original. For starters, mediocre D&B vs. epic techno.
XGRA, which amongst other things changed the soundtrack from being relatively "underground" techno to more "mainstream" music and "space-metal", not to mention the often annoying audio-video taunts from other pilots (and only the audio can be turned off!)
Caves 3 in the original. Easy to fall off of at any speed over a hasty crawl due to the near-complete lack of rails in many sections on a marathon race. The only merciful details are that if you've raced well enough up to this point on most difficulties, you can get last and still continue, and it's so hard even the AI falls off regularly.
Pasiphae 6 in Extreme-G 3 is full of nice, twisty turns to slam your bike into at 800 mph.