Magic Find doesn't boost your odds of finding everything equally. It's all the same up to 100%, but after that your odds of finding Rares, Sets, and Uniques drop steeply. Blizzard added this in since it's possible to get such huge Magic Find numbers in the Expansion, and they didn't want it to be so easy to find the really good/scarce stuff.
You can see exactly how much in the table below. A more detailed table [[Magic find diminishing returns|can be found here]].
<span id="mfpercentages">'''MF Percentages'''</span>
If you can get to 250-350% that's really all you need, in terms of finding Sets and Uniques at an increased rate. More after that generally doesn't make enough of a difference to be worth the equipment sacrifices, especially if you are playing Hardcore.
In addition to this, many monsters have unequal odds to drop the various qualities of items. Act Bosses have better odds to drop Uniques than anything else does.
=Magic Find FAQ=