I did a lot of leg squats, leg presses, leg extensions and calf extensions with weights in my 20's at the gym. I became a lot faster in a sprint and on flat roads.
But, my leg muscles were huge and my hill climbing was worse than ever due to the excess weight.
So, my experience was that leg exercise in the gym were great for sprints and flat road cycling.
It's balancing act between power/weight ratio.
Edit: Obviously, leg exercises in the gym are good for the track racing. See Chris Hoy below.
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