now i have played it for a bit, i will give my review (by which i mean complain a bit):
you tap the screen to move, and tap to select a target (you will keep attacking it until it moves or dies, which means you don't have to keep on tapping on an enemy to attack like i did the first few days). but sometimes the screen gets so busy with enemies and graphical effects that you can't really see what to tap one. occasionally there are obstacles on the road you either have to jump over (barricades) or slide under (metal shutters) which will flash up a little warning sign when you're approaching them, but if the screen has a lot going i sometimes missed the notice to dodge them.
you get magic materia, which will recharge after a few seconds when you use them. there's at least two levels to them (eg: fire, fire ii), and so far i've seen: fire (ii), blizzard (ii), thunder (ii), aero (ii), comet (ii), cure. they have different effects (fire shoots a fireball forward, blizzard makes ice spin around your bike, etc.) which adds something to the game that the original mini-game didn't have. some enemies will have weaknesses or resistances to certain elements so changing your materia can help if you struggle on levels. though thb i stick to blizzard and aero/cure (blizzard and aero both do multiple smaller hits of damage around you which is good when there's a lot of enemies on-screen). materia levels up either by combining them with other materia, or by levelling up after missions.
you can create and upgrade new bikes and weapons using materials you win after completing a mission for the first time and on a roulette after every mission. there's several spots in a circle with the items you need for bikes and weapons. depending on the rank you get for a mission you get a score, and when it reaches 100 for that mission it adds bonus items to the roulette so certain spots will have two items instead of just one. since it's random it can be a bit annoying to get the item you want specifically, and there's no way to get them besides that so far. you can get other items besides the ones you land on, but you have to spend gems and the amount you need goes up the more you select and you have to start from whatever is next to the one you landed on (so to pick the one left of your prize you need to spend 100 gems, then the next one will be an extra 200, but even if you went for the one to the right of your prize now that would be 300 gems).
gems are the premium currency in the game. the japanese one has them at 1 gem = 1 yen, with the lowest amount being 100 gems, but if you by the higher values you get some bonus gems as well. in addition to getting extra items on the roulette they're also used to recover your gasoline (stamina) right away for 100 gems.
(there is a login bonus, which for now is 200 gems per day but that will change to include gil and items later. but for now you're stuck with the roulette for items)
bikes and weapons can be levelled up to either lv 10 or 20 depending on the weapon/bike, and some have different bonuses such as extra materia exp or breaking through barricades. once you level up a bike to the max you can 'limit break' it to reach a higher level, but this requires a second bike of the same type that is also maxed so that's kind of inconvenient given that you're stuck hoping you get the item you want.
cloud has a level, which when it goes up gives you more total gasoline and fills it back up to the max. it doesn't effect your stats though, that all depends on your weapon (attack) and bike (hp and defence). to do the later missions you have to upgrade, just because you won't be able to survive if you don't have better defence and hp.
it's split into chapters (i've gotten up to 7 so far, plus one tutorial mission), each with 5 missions each. missions involve either just trying to get to the finish line, defeating a certain number of enemies (10 bombs, 10 guard hounds, and so on), or protecting a car from enemies like in the original mini-game. the last mission is a boss battle. bosses each have a special attack which charges as time passes, which if you tap the quick time thing at the right time (there's a marker telling you when so you're not guessing or anything) you can get 15% less damage. i don't feel like 15% is a significant amount, it'd be nice if it were 25-30% or something. you are forced to upgrade since some bosses will have attacks that will take your hp away completely if you don't have a good enough bike.
attacking enemies, using magic attacks, and dodging obstacles fills up the limit meter. during normal stages this gives you a 'limit burst' attack when it's full, which does a lot of damage to every enemy on the screen. during the boss battles you have the choice of doing a limit burst, or a limit break (tap the meter in the top right of the screen for limit burst, tap the boss for a limit break). you get more quick time markers during limit breaks, which give you either 0.05 or 0.10% more damage. the limit attack you do seems to be pre-set for each boss, or depending on your weapon.
you can pick a support character (so far it's only tifa), which gives you boosts like extra attack power. during boss fights they will also do a 'unison limit break' which means after cloud's done his attack the support character will follow up with their own. there is a 'talk' section on the main screen (called 'friend' in some of the older screenshots), which looks like it will be a sort of communication mini-game where you talk with other characters and build up your relationship, then you can use them as support characters. the silhouettes on the screen show tifa, aerith and yuffie. but this feature isn't available yet.
there is a 'daily mission' category on the mission screen, but that's not had anything there yet either.
currently there's a special event where you fight bahamut, which gives you materia like fire/blizzard/thunder/aero/comet ii and items to make a 'bahamut' bike. this is where i have some complaints. there's three difficulty levels, but the only real difference is bahamut having higher hp and attack power. all the moves are the same, the high level battle is basically the same as the low level one. except if you don't have a good bike (like the bahamut bike) and a good weapon, you're not going to be able to beat it. i managed to beat the high level one only after getting the bahamut bike to lv 20 and upgrading the weapon i'm using (which is annoying because they use some of the same materials so i upgrading one means i don't have the items to upgrade the other).
so basically, it's not bad but there's some minor annoyances here and there. it's one of those 'upgrade your equipment and slog on' games more than a skill testing game.
nitpicking nerd complains: it mentions shinra in the mission text, cloud and tifa have their old ffvii costums so i assume that would place it near the start of ffvii. but you have cid's picture on the customisation screen which doesn't make sense since he was in rocket town, and yuffie's silhouette is on the talk screen. but since it keeps mentioning shinra like they're still around, it can't be at the end of ffvii (also aerith on the talk screen). also it all has taken place in midgar so far (up to chapter 7) but you get some stages taking place at night, some in the evening, and some during day. but midgar in the game was always night, the sky only changed post-ffvii.
in conclusion, this game is not canon