I had a game last week where I picked Vel'Koz mid, they "counter-picked" me with Talon. My team raged at me for not letting Fiora mid since she got countered by Teemo. I simply told Fiora to farm as much as possible and Lee to prepare to gank top often. Lee told me to stfu. Fiora falls behind, meanwhile, I've made Talon my b*tch, even with their jungler (Warwick) camping me (Guy even burned his ult twice trying to kill me, the second time under my tower for a free kill for me.). Killed WW when he had blue, then started roaming to make up for Lee's lack of ganking. I head top, delete teemo, TP bot, delete their bot lane, all to the shock of every player. Talon in particular was shocked:
: How do you go from 1/8 as Tristana to 7/0 as Vel'Koz?!
: They didn't ban Blitz and didn't let me mid.
: I don't see why. You're scary as hell.
In context, the previous game, I was Trist versus a very aggressive Cait/Blitz bot lane. Kept trying to farm under tower, and Blitz would just pull me out from under it, and he and Cait would just destroy my ass. At least I know he scoped out my match history. Trust me, if the Mastery system was live, I'd already be a master at Vel.
Anyway, the Lee was bragging about him carrying our team. Yet he barely ganked, while I scatter their team just by showing up. In fact, I was so fed by the end, only Teemo was able to give me problems (Little rat was responsible for most of my deaths thanks to his poison/shrooms), and their team asked if my tentacles hurt carrying Lee.
So yeah, counters aren't everything. I've forced so many "counters" to be my toys as poke mages like Vel'Koz and Xerath, simply because every time they tried to farm, I'd just Xerath Q/W or Vel W/E to force them to back off. Then again, I get flamed in Bronze for going TP/Flash on Vel and Xerath simply to make up for my lack of mobility and safer laning. I've even made "expert" Zed and Kat players cry or ragequit. Just play who you're best at in the position you're taking, and you'll beat people who take certain champs just because they'll "counter" you. Talon here learned that first-hand. (Pretty sure the guy banned Vel in his next few games, just in case he came across me again.)
Hopefully Riot reworks Lux to be a better control/poke mage than she is now. The hatred I will generate with these three >:D Hopefully Veigar gets a little better after next patch, so that I can really be a bitch to assassin mids.
For reference, here's what I usually take mid:

And here's the game in question:
http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1743632461/36027185?tab=overview