IMHO, the activity percentages are worth the price of admission. I never allowed haste or slow spells in my campaigns, because there were no logical rules I could wrap my brain around.
With activity percentages, that problem vanishes. Haste? You can take actions adding up to 200%. Slow? You can only take actions that add up to 50%, or perform a "Full-round" action over the course of two rounds. Simple, concise, and well thought out.
Also, Arms Law contains bigger critical tables than CC does. That means you can use the CC tables to generate damage, but read the results off the Arms Law tables. (More variety)
Anyway, this is all just my opinion. I'm sure other people will have different points of view.