General opinion.
+Reduced Cost
+Can get Slow utility earlier
+Possibly in a supports budget.
-HP and AP reduced by 1/4
-flat lined the Slow stats
-Reduced slot efficiency
Predicted Effect.
With the reduction in slot efficiency, it's going to become more of a niche pickup. Meaning, your going to pick the item up for the slow, and not it's stats. And will be less likely to see in a core build anymore. On the other hand, the reduced cost makes getting the slow (though nerfed) is possible Earlier, built into more decisively, and possibly allow for some supports to buy it.
In general, Riot's direction with Rylais is very much akin to the changes to Hour glass. Anything with really good utility, get's its stats reduced, so that when people pick it up, they do so for the utility it gives them, not just because it fits into a core build.