For now, KMS caused only problems for me.
KMS does (AFAIK):
Flicker-free boot experience and "graphical" text mode - i dont see their relevance until the OSS drivers are fixed to have the same level of reliance as nvidia's.
The reduced boot time is not brought by KMS/Plymouth, but mainly by rearranging the start order of services... Also, i dont care about + 10-20-30 sec boot difference if afterwards i have a system that is rock steady and its reliable.
These graphical bells and whistles really should wait until the OSS drivers mature enough to be stable - i would say the devs should channel their energy to make better/more stable drivers not introducing new features that are purely (maybe i am mistaken here, but this is all i see) cosmetic in nature.
I am a Nvidia user but i(and not only i suppose) would really like an Intel driver that is stable and uses the 4500's full capacity - there are tons of cheap laptops out there with this card and i have to look for nvidias because those have the only stable Linux drivers.