I'm confused by everyone saying that you've already paid tax once (when earning) and shouldn't have to pay tax a second time (through IHT). The taxes are paid by different people - first by the original owner and then the one who inherits. Also, much of the value, if it derives from property, has not been taxed at all, it was probably accumulated by the sheer luck of owning a property during a property boom. Finally, taxes aren't levied so much on money but the movement of money, so talking of something being taxed once and then taxed again many years later when it is changed from one form to another doesn't strike me as logical.
Personally I think IHT is a good tax. It is genuinely progressive in that it falls more on the wealthy than the poor (as opposed to, say, VAT). You also only pay it when you're dead - I'd rather money was taken from me for the good of the nation when I was dead than when I was alive and able to enjoy the asset. And the recipient already has a huge amount that comes to them tax free. £345,000 is about 14 years worth of the average UK household's income, and you get all of that untaxed. Above that it is only at 40%, so a person inheriting an estate of £500K still gets £438,000. Personally I wouldn't baulk at receiving that much, especially as I had done nothing to earn it, except perhaps sort out the probate process. The only problem I can see with IHT is it's voluntary nature, in that it's easy to dodge.
The only people who I can see who would be opposed to IHT are those with wealthy relatives who are waiting for a windfall when they die off.