I'm a Platinum 5 Solo Queue player in NA (currently sitting at 1910 elo)
Quick facts: I love the top lane and bruisers. I especially love Garen, Renekton, Shen, Wukong, Shyvana, Jarvan, and of course, Riven.
I've been playing Riven since I started this game (Middle of Season 2), I hold a 61% winrate with her in solo queue out of 77 games. I have a total of around 200 games with Riven. I feel it is not a lot, but adequate enough to pass on some knowledge to incoming Riven players.
Pros: Manaless, innate mobility, high damage early on and good scaling for the rest of the game, low cooldowns, medium risk high reward, wall jumps, decent waveclear, high ad scaling, all ad spells, semi-high skill cap, fun to play.
Cons: melee, squishy, vulnerable to CC, no true gap closer, if behind she is pretty much done since she can't build tank efficiently, all AD so vulnerable to armor stacking, pretty easy playstyle to identify.
When to pick Riven: The jungler is tankier than you are. Your team lacks damage. Your team has a form of initiation that allows you to follow up with your damage. Your team needs a secondary carry. The enemy team doesn't have a lot of CC.
When NOT to pick Riven: The enemy team has a lot of CC. The enemy team has champions that integrate armor into either their builds or their kits. Your team has too many squishy champions. Your team has no adequate follow-up/initiaton. Your team has too many melee champions. Your team has no AP's.
Terms: AD - attack damage, AP - ability power, DPS - damage per second, CS - creep score (killing minions and jungle monsters), carry - the damage dealer of the team that is relied on for the main source of damage in order to kill champions and win games, assassin - a role where killing champions quickly and effectively is your goal, glass cannon - building purely damage items and not tank, CDR - cooldown reduction, MR - magic resist, pen - penetration, bruiser - a champion that acts as primarily a tank and less of a damage dealing role, CC - crowd control, kiting/orbwalking - the act of fending off an enemy champion by autoattacking while moving away from the enemy champion at the same time to gain distance, AD Caster - a champion that relies on spell damage as the main source of damage, in this case attack damage, Trading - exchanging damage with the enemy champion in lane, harass - when a champion hits you with autoattacks and spells in lane to trade, Poke - a champion that uses ranged spells to harass you in or out of lane, All-in - a trade in which you blow all your spells and offensive summoners in order to go for a kill, Gank - when an allied laner or jungler comes with you to catch an enemy by surprise and force fights in which you have the advantage.