I've read multiple reviews that say the guest AI is bad in this game. Specifically that they don't go to new rides, or they get confused and stuck in very large parks.

Is this something that was eventually patched? Will it still be an issue in the Platinum version?

Oh, it still can be an issue. Say your guests are really hungry/thirsty and you decide to build a few more food and drink stalls to appease them: suddenly that's all they want to do, completely ignoring the rides, even after they've sated their needs. Things don't go back to normal until they've been replaced with new park visitors. I swear I almost went bankrupt the first few times I encountered this oddity, as I wasn't prepared for it. If you can, depending on the scenario, try to advertise like mad at those times or plan ahead and build additional stalls as you go even if you're going to lose money on them for a while.

As for ignoring some rides for others, I've noticed that sometimes when I build new and exciting rides if some of my other rides aren't in the vicinity of others they may completely get ignored at times. I'm pretty sure it has to do with the somewhat similar behavior above, as it also goes back to something more normal as new blood enters the park.

It may be a bit of a nuisance, but once you know how to deal with it it's a far cry from game-breaking.