If you've made it this far in the series, give yourself a pat on the back. It takes a real trooper (or at least somebody who's obsessive about plot) to sit through the hours upon hours of cutscenes found in the first two games. Consumer attention spans already withered, Namco decided to cut off the last four or five "episodes" making Also Sprach Zarathustra the finale. You'll finally get the answers to all those questions that have been brewing in your head since you first met KOS-MOS several years back!
Episode III doesn't break tradition, though. It boasts 8+ hours of movies and cutscenes. In fact, it's several hours into the game before you even get to battle! Thankfully, they included an easily-accessible compendium for those who get easily lost in the labyrinthine plot. If you don't understand it at all, you probably gave up on the series a long time ago and in that case, it's handy for experienced players who need to fill in a few forgotten details. Anything you want to look for, you can probably find it.
One thing that Xenosaga has always been criticized for is its moribund gameplay and awkward battle system. The developers have promised that Episode III will feature a combination of the two systems from previous games, but as the old saying goes, two wrongs don't make a right. Whether the inclusion of a new "Break" feature (which allows you to fill up an enemy's meter and stun them for a couple rounds) will improve things or not remains to be seen.
Otherwise, the battle system has been somewhat simplified. Though more classic, it makes fights move faster and eliminates the sometimes overly-complicated elements of the old battle system. Other stand-bys have returned since being eradicated in Episode II, like the good old shop system and ability to buy/sell equipment and items. It may seem rote, but this familiar (re)addition helps alleviate some of the more esoteric elements of the game. Monolith also included skill trees for each character, making their development more controllable and interesting to the player than before.
And for those times when you just need to get away from the main game? Well, there's Geomix, a nice platform-inspired mini-game with 60 levels and an editor for creating your own. Sometimes Xenosaga can be very overwhelming and it's nice to have the occassional distraction.
If you're just now sinking your teeth into XS or your OCD compels you to finish off the series' famously existential storyline, Xenosaga Episode III should be shipping at the end of this month.