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.
Best Boss Ever: The boss from the fourth mission of the second game is a giant, apish, speaker-handed robot piloted by a mad scientist, and you have to use the power of Space Michael Jackson (voiced by Michael Jackson) to defeat him.
It's Short, so It Sucks : Some complaints...at least until you realize this is actually quite hard! Not to mention, some magazines mentioned that you could beat both games in one afternoon...but didn't complain about the length.
Part one had some of this considering you can literally play the game for an hour and beat it. However, there is an extra stage that's even harder.
Memetic Mutation: Ulala has sometimes been remixed into challening other characters to dance battles.
Look on playthroughs of this on YouTube and check the comments. Older comments will ask why Sega didn't make a Wii version; more recent comments will have also expanded it to include the PS3 Move and the Xbox Kinect. (Though most often the Kinect; because a Space Channel 5 Mini-game was released on the Sega-Superstar game for the Eyetoy.)
A very slight case. The rhythm is slightly "off" in the PS2 version of the game, especially compared to the original Dreamcast game. A skilled player can adjust to it, but it is definitely noticeable enough to cause problems and has been commented on by a number of players.
Then, there is the Game Boy Advance version.
Don't forget the PC version of Part 2, the timing would sometimes go off for no reason, the game crashes at certain parts (Such as Report 4) when the voices are set in Japanese and the Japanese cutscenes are missing.
That One Boss: Evila, Giant Evila and Purge the King have made many people toss their controllers.
The latter two because you have to quite literally do the opposite of what they tell you to do. And when you're so used to doing exactly what they say...you can see why people recommend holding the controller upside down. Purge the King also only gives you three chances to screw up!
Blank TV can also be irritating at times, seeing as he has no exact rhythm. Also, if you screw up, he makes it EVEN HARDER. And you don't have many hearts to screw up with this boss.
The first part of the second level in Part one. You have to rescue the space captain and the other staff members; which gets pretty hard when they send some fast commands at you. (Players recommend that, unless your thumb goes fast enough across the D-Pad, you use your two fingers)
For that matter, any level where you have to either get delayed commands ("Up.....up hey!") or where they send a ton of commands at you. (There is one part in stage 4 in Part 2 where you have twenty two commands to repeat!)
Woolseyism: Ulala's name is a localization of Urara (See Meaningful Name), which is a common Japanese name. However, the pronounciation turns it into a pun, since they prounce it as "Ooh-la-la"...which is a phrase that is often uttered by people when they see beautiful women. Conveniently this is part of the game. Fridge Brilliance perhaps?