Runs fine on my Mac, except for a strange interaction between it and the NVidia chipsets that causes it to tear video (apparently it doesn't happen on ATI-based Macs.) There's a workaround you can use with Firefox that fixes that, at any rate.
Certainly seems to suck less CPU than Flash, which can peg both cores of my MacBook Pro just playing a YouTube video full-screen.
* Silverlight doesn't run on PPC Macs
* lots of video quality issues: poor picture, blockiness
* lots of audio syncing issues: audio is several seconds behind on many videos
* can't revert back to the old player once you "upgrade" and most customers are saying that they were conned into "upgrading" by Netflix.
Finally, this all started in November of 2008 and comments are still pouring in and it's March of 2009.
I'm out of luck because I have a couple of Linux machines and a PPC Mac at home...
The previous player ran very well on a five year old Dell laptop.
I also tested it briefly on Safari 4 and it seems to be better.