OK, i finished the WHOLE walkthrough (post game also) and i feel like giving my opinion.
Very useful early to mid-game as an Oshawott and Dewott, but late-game loses a lot of its usefulness. Too slow to outspeed the IMPORTANT things, too frail to take the hits it would need to take, and to weak to have the KOs it should have. He's like trying to do too much things at a time and he fails by just a little bit in every domain.
Mid-High
Very useful at first when you catch it (you can have it just before the Bug Gym which is helpful), easy to train and has great power, great early STAB move in Fire Punch and great STAB move later on with Flare Blitz. Outspeeds many things and have access to many coverage moves like Bulldoze. Flare Blitz recoil can be annoying sometimes.
High
Since I caught it just before th electric gym, it was easy to train. Good early STAB move as well as early evolution (which comes with an awesome steel typing) are godsends. It could outspeed many things and OHKO them easily. What i found out is that it had problem with the Elite four, but for every other battle it was great.
High
Two words. Fucking. Godsend. Acrobatics is just awesome. Defeatist is barely noticeable as you OHKO everything in your path. Rock slide and Bulldoze via tms are good too, as well as great lvl up movepool including crunch and later on (just before E4) U-turn. High speed is great also, it annihilates Liepards before than can do anything. It performed well fighting the E4, especially against the fighting one which my team had trouble against.
High (btw save before reviving the fossil you don't want a timid nature).
Comes kinda late and is sort of weak and frail, so some grinding is required (it comes just before the flying gym so that part should be easy. Once its evolved, though, it really starts kicking asses. It lvls up very quickly and STAB compound eyes Thunder is a blessing. Signal beam is great when you get it (which is not TOO late) but starts getting weak against the E4 (not 1hkoing musharna after a charge beam boost is annoying), but Its still able to plow through Caitlin and that dark guy after 1-2 boosts. The bad part is that its really frail (especially against physical moves) and needs a little set up before being able to plow through bulkier opponents. Bug Buzz at lvl 60 is late but great and it really helps with the second part of E4. The efficiency of the Spider goes up as you advance through post game. Also it fucking outspeeds everything (even Ghetsis' Hydreigon) with a positive nature (i had hasty). It also gets a few cool physical moves like sucker punch off a decent attack stat, but you're better off sticking to Thunder/thunderbolt - Signal Beam/Bug Buzz - Charge Beam -Energy Ball.
Mid-High, very useful post-game.
Is a very useful HM slave with cut strength and fly but besides of that is
low.