Throughout entire Force Awakens, Finn has no idea what he's doing. He doesn't know Millenium Falcon has 2 guns, he's a terrible fighter, and he is always trying to run away. In addition he is a sanitation worker which implies he is private-level (to speak US military out-of-universe).
Does Finn have any strengths, either in movie, in canon, or out-of-canon?
EDIT: I am writing a fan fic, otherwise I wouldn't need to know this.
All quotes below unless otherwise noted are from "Star Wars: Before the Awakening" prequel book (which I will shorten to [TFA prequel]) "Finn" chapter; and Alan Dean Fosters "The Force Awakens" novelization (aka [TFA book]), and WGA leaked TFA script (aka [WGA script]).
Finn is an excellent soldier, as far as soldiering skills go, if one goes by his training record. He was personally selected for promotion by Captain Phasma.
FN-2187 was simply Eight-Seven whenever one of the team wanted to shorten his designation. They didn’t do it very often. He was, as far as the training cadre and his peers were concerned, one of the best stormtroopers anyone had ever seen. He was everything their instructors wanted—loyal, dutiful, brave, smart, and strong. Whatever the test, whatever the evaluation, FN-2187 consistently scored in the top 1 percent. So he was FN-2187, well on his way to becoming the ideal First Order stormtrooper.
[TFA prequel]
and Hux talking to Phasma:
“FN-2187,” Phasma said, “has the potential to be one of the finest stormtroopers I have ever seen.”
“From what I just observed, Captain, I agree.”
and in direct evaluation of the training excercise
At last she said, “FN-2187, your targeting was exemplary. According to the simulation, you fired your weapon only thirty-six times, scoring kills with thirty-five of those. You deployed one explosive, which resulted in the achieving of the objective and another six enemies killed.”
and
After the second multiforce battle simulation, Captain Phasma singled out FN-2187 for praise in front of everyone who had participated. She had him stand and face the debriefing—and there were hundreds of them there that time, all the pilots and stormtroopers and instructors; it felt like everyone. She talked about his skill and his efficiency and his ruthlessness, how all the trainees could learn something from watching FN-2187.
and Phasma again:
She sighed. She’d had such hope for FN-2187. He had shown such remarkable promise. He had shown the capacity to be special.
Aside from merely being a great soldier, he's noted for his leadership
“They completed the objective due to the skill of FN-2187’s leadership,” Captain Phasma said.
[TFA prequel]
He's even good at physical melee combat, not just regular infantry
When they’d finished, his arms ached from the effort of maintaining the mace and shield, but there was a sense of pleasure, too, the delight of learning something new and learning it quickly and well.
He also won 5 bouts in the fight contest, and only lost to FL trooper in bout 6 because he basically got distracted by thinking about his empathy.
Finn is quite intelligent.
He's the only one questioning Han on why he does what he does and other details.
He asked Han why they're going to Takodana/Maz's. He asked Han how he plans to reach Starkiller through the shield.
He's the only ST who was noted (and based on overall impression, likely one of the few who did - it was an insightful thought) to have questioned why they are being trained on anti-lightsaber melee weapons:
They explained the composite alloys used to make the weapons, how some of the equipment was strong enough to block even a lightsaber. FN-2187 wondered about that—not whether it was true but whether or not they would ever be expected to fight someone who used a lightsaber. According to the First Order, the Jedi were extinct.
Finn is unusually empathetic - especially considering the brainwashing he was subjected to when raised and trained by First Order.
That started showing in his training (sorry, half the prequel book is about that so I won't do any quotes or this answer will grow to 5 pages long).
Then we see him refusing to shoot the villagers on Jakku
Then he helps Poe (yes, we know he had ulterior motives too, but still...)
Then he decides not to run away with the pirates from Maz's, and instead returns to help Han.
Then he goes to Leia to ask help in saving Rey from Kylo Ren.
Ship gunnery:
While you can make fun of Finn as far as ship gunnery, remember - he successfully managed to shoot up half of Star Destroyer's armament while flying TIE/se with Poe (and that was the first time ever he had a chance to shoot from this type of weaponry). He then shot down 2 TIEs in the Falcon. Admittedly, one was luck and the other was Rey's piloting skill, but he still did it.
He managed to surprise Kylo Ren in a lightsaber fight.
Yes, he had melee training.
Yes, Kylo Ren wasn't firing on all cylinders AND was injured AND wasn't gettin' real about that fight for a bit. BUT HE WAS A TRAINED Dark Side warrior, with the Force on his side. The dude who killed all of Luke's Jedi padawans; who was able to freeze blaster shots in mid-air and presumably, wasn't all too shabby with the lightsaber given his genetics.
He managed to get Rey to call him a "friend". That was a near-impossible task, at the best of my knowledge she never acknowledged anyone else as a friend, ever (even BB-8)
From what the movies (TFA and TLJ) show, Finn is moderately good at fighting (especially with blasters). His only other strength is his knowledge of the First Order. He is also decent at blocking bladed weapons.
