Deposit plus can be interesting but only if you actually want the other currency you are linking it to. For example, if you are already sending USD, GBP, AUD, etc back home every month, or if you are planning a trip to Japan and need JPY in the next few months, etc. Also, you cannot be in a hurry to get the other currency, because its like a game, at the end of the deposit period you dont know if you will end up with HKD or the other currency you have linked it to.
For example, you can invest HK$10k for one week (or one month, etc), linked to USD, and get a guaranteed 10% interest rate. But what is not guaranteed is that you dont know if you will end up with HKD or USD at the end of the week, depending if the fx rate has gone up or down from what it was at the start of the week.
As I said, if you end up with USD and anyway you need USD for something and were already planning to do some USD conversions, then it can be good because you get your 10% interest. But if you end up with USD and you have no need for USD and then need to convert it back to HKD you are screwed.