I don't like Pokemon Collosseum at all and think it aged like milk compared to XD, which is still very playable today.
Let's start with the positives. Pokemon Colosseum is a Pokemon game.
lol
Now for the negatives:
This is a very difficult game. The bosses are less of a fun challenge and more along the lines of "lol let's spike 5 levels AGAIN from the regular stuff." This is already a tedious enough game since literally all you do is battles. It didn't need forced level grinding.
All of the bosses are also designed to irk the heck out of you. Miror B. may be iconic but nobody likes the actual battle where you have less than a handful of SE options and stall tactics. I mean, yes the music but good music can't save bad design. They can't even kill you fast anyway, making for a very boring battle.
Everyone knows about Dakim's EQ killing Entei, but other than that he generally has some strong Pokemon. His team, despite some Protect prediction, is actually fairly exploitable due to 3 quad weaknesses and at least you can finally level Shadows before him. Mostly a fair, fun fight.
Venus is just...
status cancer.
But Ein is even more annoying
Have fun exploiting those weaknesses when Confuse Ray is on half the team.
And then there are the issue of the final bosses. Nascour isn't that bad but its annoying as heck how if you lose you have to do the four filler fights again. Evice is by far the worst and hardest final boss in the franchise BY A LANDSLIDE, he takes the level spike issue already prevalent to an unreasonable degree (level 60s when you've been fighting high 40s/low 50s), you HAVE to go grind to beat him if you're playing normally. His Pokemon are good when 80% of your options are just pure crap, it's not fair or good game design in the slightest. Seriously, look at this team and tell me it isn't Gen 3 competitively-oriented at least somewhat:
It's not fun at all, makes the game slow to a crawl worse than Sinnoh ever did at the end (which is saying something considering Cynthia's notorious reputation). It makes me actually appreciate Diantha! I mean Diantha may be "literally who?" and easy but it's much better that I can beat the game without wanting to snap my controller in half.
Many of the interesting options fail to reach their full potential mostly because the physical / special split hadn't happened yet or they need absurdly late / hard to get TMs like Sludge Bomb for Qwilfish. Yanma, Dunsparce, Sneasel, Sunflora, Delibird, Aipom, Remoraid at level 20 lol - there are so many borderline useless options, in contrast to XD, where even garbage like Spinarak gets Signal Beam. Even most of the usable Pokemon in Colosseum have virtually ONE move they are good at using - Slugma with Flamethrower, Ariados with Sludge Bomb and that's about it. Diversity? Sorry, that's for
other games (yes Octillery, but again, level 20 when you start the game at level 30). Not to mention the overabundance of Water types at the expense of other Pokemon types, which usually get a grand total of like 1-3 options.
The dark tone isn't much to write home about either. They wasted the most interesting protagonist we ever could have had by making Wes mute, like, what was even the point of the opening cutscene if you're going to skimp on characterization??? I mean, yes Michael from XD is a kid and also a mute, but that also ties in with the rest of the series better. Sure, Wes may be cooler, but they did nothing with him. The villains are neat admittedly but the actual fights sour my opinion on them. Not to mention the whole "dark" aspect isn't even unique anymore thanks to Gens 5 and 7.
Not a negative, but Pokemon Colosseum does have some pretty good music, same with XD. Tsukasa Tawada deserves more games to work on, amazing composer!
So yeah. Pokemon Colosseum aged terribly and I think it's one of the worst games out there, like I even like HGSS more. XD is so much better balanced to the point I'd tell you to skip Pokemon Colosseum entirely and just play XD. Sure the last couple areas drag in XD but at least I can
finish the game.