Just as mentioned in another review, if you're using the compare with price option on shopify, the strikeout that's normally used to identify the regular price is removed when the currency converts. This is definitely confusing for the customer!!
Additionally on the free version the "Customizable decimal formatting" means that either you have it set to "currency default" where XE determines how many decimals are 'normal' or a default number of decimals of your choosing for ALL currencies. Probably not problematic for most but when you're converting from a currency that doesn't actually deal with decimals / cents because they're already in the thousands with USD, where they like their quarters, it just doesn't make sense.
Obviously you can pay for individual rounding options but the free decimal option just seems a bit useless unless you're dealing with similarly numbered currencies like Euros and USD.