There is every reason to get an Octopus card as it is very handy, for public transportation which is very clean and efficient in HK. This card can also be use at many outlets such as McDonalds, starbucks and 7-eleven, so you need not worry about loose change etc.

Each person must have his/her own Octopus card.

As for going from Airport to Conrad, the airport express train is great but you will need to take a taxi from Central to Conrad, which is HK20 (US 2.5) and 10 minutes. Journey time will therefore be 25+10 min which is great. You can also take a bus that goes directly from airport to Conrad area for about HK30 (but I am not sure of price or its number, something like A16 or A40). The most relaxing, in my view, is to take a taxi, which is about HK350-400. Even a pre-ordered Mercedes costs only about HK500.

Cheers.