The Elder Scrolls Online is a very capable MMORPG, and one that does offer a good challenge to other titles in the genre. Thanks to the Elder Scrolls features it injects into the MMO mold, it plays and feels very different to other major MMORPGs, and the PvP is very enjoyable, adding a whole new spin to the game. It's got the in-depth features we've come to expect from persistent, open world MMOs, and it's got a lot of potential yet to be touched on. With some changes to the convoluted and restrictive guild system, it could really shine.
With that said, I did find the PvP design to work very well, and the idea of fighting for control of keeps and castles to bolster your member's skills and acquire rare materials is interesting. The Alliance War back story is used to explain this, hence the need to choose your allegiance when you create a character, and the way PvE and PvP is separated makes more sense than simply having random zones where PvP fights can take place. It keeps both modes in line with game and flow together as an interwoven narrative.
The PvP zone isn't just a simple battlefield, either. Instead, it's the whole province of Cyrodiil, complete with it's own cities, settlements, and quests. You can wander around it taking on missions, only here you can run into hostile players as well as other AI foes. This means you really need to be prepared before you head on in (which you cannot do until you’re at least level 10), and a mount is nigh on essential so you can flee from battle if you need to and traverse the long distances you need to cover.
You also need to ensure that you have a group of other people to take on enemy positions, as going solo isn't really an option here, no matter how stealthy you may be. It's all too easy to be overwhelmed by a group of players, and backup is needed, along with siege engines, which are used to attack the enemy, and to damage their fortifications.
I enjoyed the PvP, and found it to be a lot more interesting than many other MMORPG efforts, and there are a lot of campaigns on offer, each of which run for a set length (some as long as 90 days). Winning these grants members of the conquering alliance buffs and bonuses, and you'll also still level up, so your efforts carry over to PvE, too. Even doing the PvP tutorial gives you a skill point, and Cyrodiil also has Skyshards just like the PvE areas (collect three to get another skill point).