Why Progressive springs are a bad idea
When BrianM was working an AMA race bikes for Traxxion Dynamics they tested some Progressive brand springs. Progressive is the most common brand of progressive rate springs, but their results apply to any brand of this type. The Progressives are really light for the first inch (like .45), about on par after the 2nd inch (1.05), and by the 3rd inch of travel are so far beyond reasonable (1.7) that you'd never get any more travel.
To make this simple: They don't work because they go from WAY too soft to WAY too stiff in a very short distance. Get straight rate springs.
Progressives limit the amount of work the forks can do; it's the suspension equivalent of installing a speed limiter at 60mph, which doesn't make sense. You want to use 99.99% of the fork travel when you ride your hardest (be that emergency stops or corners)... With Progressives, you'll be lucky to use a Total of 2.5", or only about half the total travel.
This all means that in more extreme situations, when you NEED your tires on the ground, the springs are going to fight you. Run over a 2x4 (that the suspension should soak up reasonably well) and you'll have the front wheel in the air. Hit a pot-hole mid corner and not only will it be fairly jarring, but the fast rebound will reduce the weight on the front tire and increase the possibility of lost traction.
Progressive brand springs also have a bit of a problem with quality control. Traxxion found a couple springs that were drastically different from one another when pulled out of the same forks for service.
Progressives cost about the same as normal straight rate springs. There's no reason you should get them. Riders only know the best they've ridden, so lots of people will report being happy with these fork springs. But if they felt the difference between the Progressive setup and normal springs with the correct spring rate, they would agree that Progressives aren't worth the money.