I stayed at the property a few weeks ago ... a few comments.
As a platinum you will be treated exceptionally well. As mentioned you will have automatic access to the lounge (which is open on weekends). You will also have the option of using the Market Cafe downstairs for breakfast - complimentary. Both are excellent. The lounge is pretty much open continuously for snacks, drinks or a place to chill for a few minutes.
The property is beautiful - and well worth the 'inconvenience' of the 10 minute ferry shuttle to the Saphan Taksin pier and the BTS. Very relaxing and away from the crazyness and bustle of BKK.
Be careful of the 'upgraded room' option as I have heard it mentioned in other threads that the upgraded rooms (could be the Junior Suite you refer to - better check) do not have a great view of the river - some are garden facing. Getting a room with a great view of the river is most important in my opinion as the city revolves around it. Try and get a corner room facing the river and you will have a wonderful view of the Chao Phraya.
The round trip taxi from BKK airport was not expensive - aroiund 300 - 400 Baht if I remember correctly.
The Manohra cruise is to be recommended - if nothing else it is a good way to get oriented with BKK when your first arrive.