People don't seem to be mentioning this, but I find it to be as bad, if not worse, than accepting plane rides and fishing trips, or a house for your mother. The dollar amounts sure are higher:
"2 Supreme Court justices failed to recuse themselves from cases involving their publisher after receiving large amounts in book advances and royalties"
A 3rd, Jackson, has accepted a million or more bucks, but hasn't had a case come before her yet. But I think just getting a million like this with the incredible potential of a case coming to your court is problematic.