Reading comments from a lot of guys on this forum about optics are pretty humorous and it's obvious that they don't do any long range shooting. That said, there are also lots of guys on here that know what they are talking about, but you have to wade through the bs.
Now, it all depends on what you are doing with a scope. If you are going to go sight your gun in, and go hunting and shoot a deer or other animal from 0-300 yards, and you just need a scope with good glass, that is durable, etc., you can get plenty of scopes in the $2-400 range that will do more than you ever need. Heck, many cheaper Simmons will probably do what most people ask of them as well, but I'd prefer to have the piece of mind and slightly better glass of the $2-400 scopes.
People on here seem to argue quite often that low light performance is a reason to spend extra. I don't know where they hunt, but where I hunt, while some scopes may be better than others, even cheapo Simmons and Tasco's will let me see well past legal hunting hours. Scopes like the Burris FFII, Zeiss Conquest, Nikon Monarch, etc will all also let me see well past the legal hunting hours. In a hunting situation there is no reason that you have to spend more on the scope than the gun, unless your gun is a $75 gun.
Where you start gaining performance past those scopes is in the accuracy and repeatability of the tracking. It's obvious from most of the posts that 98% of the guys on here that tout Leupold scopes, 3-9 power scopes, etc., don't shoot long range, and don't adjust their scope very often. If you are shooting long range you need a scope that tracks accurately and this is what you pay for. So if you are looking for a scope for a long range rifle, it is definitely worth the extra money. As mentioned before, keep in mind that just because you have a $2,000 scope doesn't give you skill to shoot 1,000 yards.