I just got viktor recently and he is really fantastically fun to play. He has the only targetted-line spell in the game other than Rumble's ultimate, and it can be used way, way, way more often, making him very unique and interesting to play. No matter how fast you are, casting it locks you on your movement path for a split-second, because you need to gesture the mouse instead of just pointing it. I tried to barely twitch the mouse to set the direction, but it's not remotely as accurate and reliable as a decent chunk of distance - something like 1/2 of the path distance. This means you actually have to weigh the likelyhood of needing to change your movement order in the next 0.3s along with everything else while you're charging forward trying to get into poke range of your laser - the opposite corner of the screen for your mouse to be from where you can issue a "move back to safety" order. This makes for much tenser poking gameplay than most characters who can easily move the mouse across the screen during the casting/attacking animation for their poke, with no delay at all.
I'm not very good with Caitlyn so I'll avoid specifics, but it largely boils down to AD and APen. AS is great but not nearly as great as you might think based on her passive, because most of her damage is gonna be in Q and R. Getting AD fairly early is essential, because while maxing Q gives it OK base damage, her ultimate has absolutely crap base damage and relies entirely on you building up bonus AD in order to do anything. You may find some extra mana useful in order to spam loads of Q on people for harass, farming, and sustained damage; in that case, just a Manamune should be plenty.
