Investing in foreign currencies

A lot of my friends and colleagues are expats, typically with currency exposure between Hong Kong dollars and their home currency. Some may have financial commitments in multiple currencies, others may be planning to retire in a different country. 

What has surprised me is that as retail investors, a number of people are actively using exchange rate movements as a means of investment, either in the form of direct buying/selling of currencies or through products such as foreign currency linked structured deposits.

Up to now i haven't been actively considering fx as a means of investment, other than looking to opportunistically exchange money when rates move in my favour.  This is mainly because i don't want to get stuck on the wrong side of a transaction with a currency that i don't want or need. However, to the extent i'm indifferent between holding two currencies, there's no real reason why i shouldn't apply the basic investing rules of looking for value & buying low and selling high to the fx markets.

I don't anticipate this to be a significant part of my portfolio, but where opportunities arise i might look to take small fx positions from within my cash buffer.