Addictive and involving.
Introduction: As far as I know, this game is an almost-perfect port of the PS game Valkyrie Profile. There are a few small changes (detailed at the beginning of the sole FAQ on this site; read them if you really want but there's nothing big changed) but otherwise the core mechanics, story and general feel of the game are the same. The big change is the addition of a small but significant number of CG sequences to replace slightly inferior cutscenes in the original, but I will go into these in more detail in the graphics section.
The game is part of an almost-series by Tri-Ace, arguably the number 2 Star Ocean game (the first was, logically, called 'The Second Story' and the third is 'Till the End of Time') although it is radically different in the way that Final Fantasies are from each other. I have played TtEoT but not TtS, and I notice the odd link between the two (such as healing items restoring a % of max health rather than a fixed amount, and two optional very tricky bosses in TtEoT being Lenneth and Freya), but the story, setting, mechanics and characters are all different.
Gameplay: This game is different to many other games in that all the characters that you can get to join you are either dead or immortal (the vast majority simply dead), and that in order to have any sucess at all in the game, you must send away at least one character per chapter (there are eight) to Asgard to fight for the gods against the demons. Thankfully, this doesn't have to be your best player, just sending someone that meet Freya's requirements will grant you the best items. Because the war in Asgard is proceeding as you are down in Midgard, entering towns (to recruit) and dungeons (to fight) causes time, called "periods", to pass. In normal mode (the best for beginners; Easy is actually harder!) you have 24 of these per chapter: always plenty of time to get done all you need to do. Even so, needless entering of towns and dungeons can really mess up your careful strategy for the chapter.
Battles are fought in a turned based and real time system, whereby you bump into enemies on the 2D dungeon map and then you get a turn, where you can attack by pressing the button corresponding to the character you want to attack with. By pressing these buttons at the right time, you can combo with other characters,building up the hits chain and energy gauge. Each hit adds an extra percent damage, so the 51st hit does 150% of normal damage. When your energy bar hits 100 you can perform a Purify Weird Soul move, an extra powerful limit break, if you like, that knocks off huge chunks of health from the enemy. Thankfully, you can practise your combos and PWS's from the party menu, taking no damage.
There's more I could say about the gameplay, but I will just briefly talk about the dungeons. You move through them in a 2D way reminiscent of Castlevania, using crystals and jumps to help you on your way: many difficult to reach treasures require careful use of crystals, crystal shards (don't worry you can make unlimited crystals, but three at a time only) and empty throwable chests; these can be quite tricky even with a guide!
Finally, difficulty: from the beginning up to Lezard's tower in Chapter 4 the game on Normal mode is what I would call easy: most battles can be won without too much sweat, but in the Tower it gets a lot harder, keeping you on your toes with, for example, two of the boss from the last dungeon with two other equally enemies. It suddenly became even more enjoyable for me at that point!
Story: I won't have any spoilers, but it's based around norse mythology: you must collect the souls of brave warriors to fight the battles in Asgard, you transfer Einherjar (these souls) to Asgard and get the game's currency and items depending on how suitable they are, but you can use them again in the final battle at the end of the game. The story is very interesting at the start, but the main story does not crop up again till the end of chapter 4 really, but the little side stories behind the Einherjars' deaths are interesting.
Graphics: Very nice backgrounds, detailed sprites looking a little off at times (Lezard's laugh, anyone?) and very nice portraits in the cutscenes. As mentioned before, the game has a few (usually fairly short) FMVs in it from time to time, and these are always richly made, with great sound, and can be viewed from the main menu too.
Sound: Music is on the whole very good and suits the game well, voice acting tends to come in arbitrarily during certain cutscenes but not in others, the voices in the FMVs are good though. The battle voices are good, suit each character well, but you will tire very quickly of "To my side. my noble Einherjar!" and "It shall be engraved upon your soul! Divine assault: Nibelung Valesti!". The bgm, sfx and voice volumes can all be adjusted but somehow it's never quite right: you will turn the volume up to better hear the voices and then be deafened by the twanging of Valkyrie's bow. All in all, good sound, but it's the game's weakest point.
Play time/Replay value: For a start, the game has three different difficulty settings and endings (the two aren't linked), but one of each is not worth the effort (easy is harder than hard in it's own way and the worst ending is totally shoddy). Playing on Hard will be much different to Easy, as there are more characters and more dungeons, longer chapters but each Einherjar recruited will start at level 1... recommended once you have played through the game at least once and you know the was through it. Play time: it will last me about 40 hours I expect to do it once on Normal, and Hard takes longer...
Final thoughts: This may well be the best RPG for the PSP out there, recommended to all fans of the genre at all, but it's a game to buy and keep, for there's lots to come back to later.