I used to be a 3rd party supporter myself.. that is until I bought my first Canon normal (100mm F2.8 Macro) and L (135mm F2 L) lens. The difference was big enough to get me hooked on original (unfortunately L) lenses. The difference is not so much in sharpness but rather in build quality (even compared to Sigma EX), AF (even compared to Sigma HSM), colour (my Tamron 28-75 looks flat, Sigma is yellowish) and contrast.
Don't get me wrong, I'm not saying that 3rd party is terrible and CMI but there is a noticeable difference although it might not always be worth the price premium. If you can't afford the originals, the 3rd party ones can still give you great results (I still regularly use my Bigma).