The dies are quite expensive, but if you buy versatile ones that you'll get a lot of use out of, it's worthwhile. You can then also sell the cut out shapes on ebay (although you don't make much money on them and you have to spend time cutting them out).
However, if you want a specific shape for a single card you may not get much use out of the die, so you might be better buying pre-cut shapes.
The companies that make the dies are wise to the ways of crafters though, and regularly discontinue dies, which then bumps up the price when people want to buy / sell on ebay.
Something that annoyed me recently is that I bought a die for making my wedding invitations, thinking it was A6 size. It's not, it's fractionally bigger, so I can only get two out of an A4 sheet, not four as I thought. Therefore I cannot fit the die cut shape on a card cut from half a sheet of A4. This means I am only getting one invitation from each sheet of A4, and a lot of waste. But, that's my personal bugbear, just something to bear in mind.