While they are not designed for aggressive riding, they are very reliable, and have a broad smooth powerband. Basically, these bikes are designed for the casual rider/beginner. When compared to the performance of an MX bike, or the WR/CRFX bikes, then there is no comparison. To say that they suck would need to be based on the intent of the rider, and is clearly an opinion. I have seen a TTR125 ridden quite hard, and while it cannot keep pace with my KX250, under certain circumstances, I would actually prefer to ride the TTR. I have also entertained the option of purchasing a TTR230 for my wife, who is not nearly aggressive enough to ride an MX machine. I am also considering the purchase of a TTR125 for my daughter to ride around on.
I did recently see a kid riding a TTR230 on a local MX track absolutely ripping the place up. He was clearing a 40ft double without missing a beat. When I realized that if was a TTR, I was in shock.