Extremely unwieldy!
I am fairly accustomed to many types of calculators and converters. This is simply way too difficult to deal with - too slow to get anything accomplished. It reminds me of those local sites that still use the drop-down box listing every country in the world, in spite of have customers in no more than three. Hack it together and hope people use it! Delete!
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Converter: Units & Currencies, v1.2.3