Project X Zone contains characters from various franchises owned by Sega, Namco Bandai and Capcom. As you may expect, there's not that much in terms of story holding it together; character's emerge from portals that connect various worlds and parallel universes. You can learn a bit about other franchises that you aren't familiar with, as well as using characters that you know and love; it's a decent history lessen. There's plenty of dialogue and a word of warning; it does contain a lot of sexual references.
Project X Zone is an turn based strategy RPG, but it is a lot more accessible for those who aren't familiar with such games. There is no player/enemy phase and instead, characters take turns according to their speed attribute. Characters are paired up and this cannot be changed. Some are quite natural pairings like Ryu and Ken from Street Fighter, or completely unrelated like Frank (Dead Rising) and Hsien-Ko (Darkstalkers). You can use as many items as you like, use an ability, move a distance based on the character's movement range. Once you're within range of an enemy, you can engage them in battle.
When the battle is initiated, you have to press buttons to attack in a simplistic system. Your attacks are A, Left A, Right A and later on you unlock Up A and Down A. If you use all of the attacks, you are awarded a "+1 attack" where you can attack one last time. If you position your character next to another character on the grid, then you also can press the R shoulder button to call them in. There are some extra characters that join your party called Solo units. If they are assigned to one of your units, you can call them in using the L shoulder button. If you attack whilst you have called in a Support or Solo attack, then you activate a Cross Attack which allows you to fill your XP gauge over the normal limit of 100%. XP is not to be confused with EXP which is experience gained by defeating enemies.
XP allows you to use abilities before you attack such as increasing attack/defensive points, heal units, extend your movement or attack ranges etc. The XP points go to your group and not individual teams. Once you have at least 100% XP, you can activate a special attack in battle to do huge damage. Use this to finish off an enemy and you are rewarded with a 20% bonus to EXP. Outside battle, you can use a multi-attack which does large damage to multiple targets in the area.
The object of combat is to keep the enemy in the air, so in theory, you should try and figure out which attack combination gives the most success and most XP. Hitting them just before they hit the ground will score a critical hit, but using Cross Attacks holds the enemy in place and thus is impossible to achieve criticals. In reality, it doesn't make that much difference, especially when the game progresses. Earlier on when you don't have many attacks, gaining XP appears to be quite hard, but later when you get more attacks, gaining XP is rather easy so it becomes a bit more mindless.
Enemies have an equivalent XP gauge, and boss characters can execute special attacks to deal large amount of damage. If an enemy attacks you, you can use your XP for countering (which lets you land a few attacks, but not call in Support characters), Defend (which reduces the damage) and Full Defend which negates all damage. There are four status ailments: Bind (which prevents the use of items or skills), Down, Stun (can't attack or counter) and Poison (lose HP each turn).
Even though the game initially sounds complicated, it doesn't have much to it. If the game was only 8 hours long, it wouldn't be too much of a problem, but it's around 56 hours long! When you begin to tire of the game, the levels just get progressively longer which means reaching the end is a huge chore. In other RPG's, a long game may feel a chore, but the story can really drive it; but that's pretty non-existent in this game.
The game-play forms a decent basis but the game sadly never builds on it, or throws twists to make you think and adapt. It doesn't even really get harder, just throws more enemies at you. If the game introduced new elements to think about and make you play more tactically, then it could be brilliant. Games like Fire Emblem have drastically different units so you have to move in formation to protect your weaker units, in addition to considering their weapon strengths/weaknesses and attack range. In Project X Zone, each unit is virtually the same as the next. Fire Emblem has different terrain types which changes your movement range and statistics, and often you have to utilise the map to your advantage like using choke points and hold your ground. Sadly, this doesn't happen in Project X Zone.
The lack of variety or interesting battle mechanics means the game becomes a chore early on, and so you end up forcing yourself to play the game. It's definitely a great time-waster but that's all it is.