I've found they often don't work on my PC in the office, but will display when I try them on other machines on the network.
Might be if the machines are on the same IP address it only allows so many adverts to be displayed in a period of time, say 5 adverts to the same IP within a minute for example. I am however purely guessing.
Then again, might just be that I need a flashy new P4 3Ghz machine....now to convince the boss!
Forgot to add, you can control the colours now pretty well - seen some really well integrated ones around although you still have the "Ads by Google" link which gives it away.
Trev