Apparently it's very team-oriented, but can be played with AI companions. Is this actually worth it? AI can never replace real humans, so I don't want to waste my time seeing as the Prophecies campaign is supposedly the toughest of the lot.
There's two types of AI companions, Henchmen and Heroes. Henchmen have preset skills and equipment (auto leveled and all that), and are good enough to get through just about everywhere except elite areas, but you'll have to learn how to handle things proficiently. Heroes have to be skilled (skill unlocks are account-wide type) and equipped yourself (armor auto levels), and are very overpowered when you use them right. Humans are even better, but even when Guild Wars was populated it was hard getting together a crew of people willing to synergize, so in most respects heroes are the top of the top. Heroes can be used everywhere but you'll only be able to acquire most of them in Nightfall or EotN.
Prophecies isn't the hardest campaign. Factions is IMO harder, along with being much shorter so that you have less time to learn the basics and get skills/armor up to par. Of course using a fully equipped Hero team turns both into a cakewalk, so you might not want to do that (or take max 1 or 2 heroes, you probably can't skill more then that well). Also I recommend taking Henchmen for Prophecies/Factions simply because a huge amount of story gets told through them later on, and its actually decently written and nice to watch as they go through the same personal quests you do (or get killed, as the early ones tend to do), unlike in GW2 where the writing is pure shit. Nightfall and EotN OTOH are balanced for a minimum of 3 heroes with you at all times and henchmen are basically an afterthought. 3 Heroes used to be the limit but it was raised to 7 as the population thinned.
Also note that there is a Hard Mode which can eventually be toggled in any area (can't remember conditions to unlock it). It is designed strictly for level 20 characters and with teams of 3+ heroes, even in Prophecies/Factions where you can't normally get heroes. I once did a self-made challenge of completing Hard Mode with Henchmen and god damn its punishing. Arguably some of the story-extension areas that were released right before GW2 to wrap up the whole series can count as elite-level content as well, and shouldn't be attempted without heroes. Especially the Factions ones.
Anything else I need to know about the game? Do people actually still play it? I've only seen two other players thus far.
Prophecies is by far the longest and most drawn-out campaign with the most number of cities, so anyone else playing the campaign is spread really thinly. Dunno how it is now but if you want to find people check the major cities or the zones for elite content. There's also the Zaishen isles which has a zone that feeds daily quests.
There's tons of other stuff you need to know, but Guild Wars is one of the best games ever in terms of exploring the mechanics, so I wouldn't dwell too much on trying to find tips. I think I had something like 2500 hours in it, amazing games.