In battle you can bring up an action bar for each Hero that gives you access to their skills and a few other controls. You can move them around either individually or as a group by placing flags on the minimap or the play field. It's nice to have this added control over NPCs, and it adds an entirely new dimension to the game, even though you have to rely on the AI a lot because it's very difficult to manage more than your own character and a couple Heroes in real time.
Another nice thing about Heroes is that they are a great way to explore the multitude of skills and builds in Guild Wars without giving up a character slot.
One curious aspect of Heroes is that they do tend to encourage solo play, which is a little odd for a multiplayer game. You can complete the entire Nightfall campaign with only Heroes and henchmen if you so desire. Assuming they are at the keyboard, I think even novice players are better than the AI-controlled Heroes, but Heroes are reliable, and they never complain if you suddenly go AFK for 20 minutes.
The storyline in Nightfall is a little less linear than those of the earlier releases, and choices you make at certain points will lead you down a different mission path. It's not as freeform as some games, but they've opened up the zones somewhat so it feels less like you're on rails. There are also unique missions associated with the various new Heroes.
I've heard some griping about the need to earn some faction with the Sunspears at an early stage in the campaign before you can progress through the central missions. This involves killing a good number of corsairs or demonic creatures, but you can also earn the necessary points by completing side quests. Still, it's not the most interesting time for experienced players that bring across a high level character from one of the other campaigns.
PvP Hero Battles
Guild Wars makes PvP much more accessible than most games of this kind. You aren't required to spend a lot of time leveling a character if you don't want to; you can create a PvP character that is already at the level cap (20) and jump right in to the arenas. You also have access to decent equipment, although you can unlock extra skills by completing the game's missions.
While the second campaign, Factions, added more to Guild Wars than Nightfall for PvP fans, Nightfall does include a new form of PvP. Hero Battles are matches between 2 players, each of which takes 3 Heroes along as teammates. The objectives in Hero Battles are captured by proximity, so you can send your Heroes to take over a shrine while you concentrate on other things. Intricate strategy is involved both on the battlefield and beforehand selecting a build for your team. There are currently 4 maps available for this type of play.
I don't expect Hero Battles to become the most popular form of PvP in Guild Wars, but it's definitely an interesting addition to the mix.
Of course, the main PvP attraction remains the guild competitions and tournaments which are common to all three campaigns. As you can imagine, the upper tiers of these contests have grown extremely competitive, and the top teams are truly well-oiled machines. If you hope to play at that level, you'll need a good guild, voice chat, and plenty of practice. Spectator mode is a great way to get some idea of what tournament PvP entails.
Guild Wars breaks the RPG mold in many ways, and does it with style. Henchman and Heroes make it very easy to sit down for a brief session that is action packed, whether you want to take some friends with you or tackle it yourself. It's a dramatic change from spending half an hour looking for an eligible healer to fill out your group as is often the case in other games. If your primary interest is in PvP, you may want to start with the Factions campaign, but otherwise, Nightfall is a terrific place to begin your adventures in this series. Veterans of the earlier campaigns will also get good mileage from Nightfall as long as they're not expecting something dramatically different. Guild Wars continues to forge it's own path among online RPGs, Nightfall is another beautifully implemented part of the package.