Pros and cons of progressive suspension?
The idea makes some sense: progressive springs being softer for smaller bumps and preventing bottoming out by becoming firmer as they compress. 

However, that is the main downside. The spring is too easily compressed at first, making the bike front (or rear) go down more easily, returning back more slowly in the "top" range of shock absorber movement.

As the shocks compress further, they become noticeably stiffer, compressing less and rebounding back faster. 

It is clear that such springs make it impossible to set up an optimal compression and rebound for any riding terrain/style/preferences.

Compared to optimal rate linear springs, the small bumps will feel a tad smaller - since the springs will work in their lower rate range. But the bumps are small to start with. As you get to the "rougher bumps", you end up with shocks that are "harder" than optimal - compressing harder, rebounding quicker. 

It boils down to personal preference: do you want even softer "small bump ride" at the cost of a bit rougher "big bump ride", with somewhat worse control/handling (compared to linear rate springs)? Progressive springs provide that.