I hate Snipers... but its hardly because I cant aim. Yes, getting killed by Snipers is annoying but then again getting killed by a lot of classes is annoying!
I hate Snipers because all they do is park themselves at the back somewhere and kill. Theyre not defending or attacking, theyre just trying to get a decent score. I very rarely see one defending a point or capping a point unlike every other class.
But that's the beauty of TF2!!! If they aren't assisting teammates and are just sitting isolated on their own, then just go spy or scout and they have no chance.
A sniper is obviously never going to be the first one on to the capture point - but that doesn't mean they aren't helping their team to attack/defend.
A sniper that is playing for his own score is usually very easy to kill. A sniper that is playing a proper team game and assisting his team mates is often a very frustrating opponent.
TF2 is a team game and you'll always come up against a frustrating opponent, no matter what class he's playing as. But there are so many things you can do to overcome this. As an individual you can swap classes to one that is particularly suited to killing that opponent (for example scouts, spys and snipers are good at getting revenge on a sniper). But by far the easiest way to get revenge on someone, is to play as a team. No one individual can do anything to stop a good team.
A sniper who sits at the back and only thinks of his own score will only end up with a good score if you and your team play like a bunch of idiots and walk out one at a time into his target area.