I tried to get into the the JP version with an english patch, but was not successful, but...
From my understanding the skill and weapon system is that of the PSO1 with an advanced combat/combo system based on PSU/PS Portable, and the leveling system based on the DS's PS off-shoot.
So it breaks down into: you can use any weapon on any class (so long as you meet the min specs) and as you level-up you get to spec into any one of the classes to enhance the stats used by that class; in combat you do more damage the farther into a chain you go, and you can chain spells (supposedly, haven't been able to play).
As far as the story goes, my vauge understanding of it is that we are many generations after the original two colonization ships in PSO1 and on the opposite side of the universe from the events in the PSU/PSP/PSP2 chain (so no beast-race).
Oh, and there are MAGs, not the stupid PALS.
As far as buying a Vita just for this... its what I did, and I'm getting sick of waiting for news on a release date. There are a few good Vita games holding me over (Raganrok Odyssee being the main one). I bought my Vita expecting compatibility with my PSPGo game collection, and I was sadly mistaken, only 3 of my PSP games have been Vita allowed (and not the important ones like Valkyria Chronicles or PSP2). So my suggestion would be to wait at least until a release date is made or if you find a must-have Vita title, or a price reduction (probably gonna be reduction shortly after the Feb. 20 Sony event/announcement.)
I am currently playing PSO2...it's rather easy to get into the beta...you just have to complete a captcha in japanese, which can be tricky.
Combat is unlike both PSO and PSU in the fact that you use "Mouse 1 & Mouse 2" for attacking rather than the direction keys. The game plays like a third or first person shooter and you can actually change your in-game view to reflect this. Photon Arts are "chained" into combos. For example, you can set the greatsword PA whirlwind to combo slot 1 which means you can either use whirlwind by pressing mouse 2 at any time or by using basic attack (mouse 1) and then "chaining" in whirlwind immediately afterward. You can have up to 3 PAs slotted like this but a PA in slot 2 or 3 can't always be used on demand, depending on the weapon you may have to combo into them. The gunblade changes forms so you must combo into PAs aside from slot 1. This is only for melee weapons, btw. Guns and Staves follow a different set of rules.
Each class has their own form of a dodge-roll, you can guard with certain weapons, and rangers don't consume PP with standard shots. You can also attack in the air and hit airborne opponents while grounded.
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