Yes, agree that the St Regis location is fantastic, right by the BTS too - even the short walk down the soi from the Hansar is a bit annoying, especially given the lack of a proper footpath.
Yes, that building is partly residential and partly the hotel. Most of the regular suites are in this 'residential' part of the hotel, with the views across the club, while the larger suites are in the smaller building (though I don't think the hotel itself goes beyond the top of the smaller building). I did a trip report about it here:
http://www.australianfrequentflyer....-and-trip-photos/tr-mel-bkk-mel-jq-29080.html
It's a different kind of hotel to the St Regis and others - more of a boutique hotel / furnished apartment. Some of the larger / vertigo suites have some really fantastic design elements to them, and they all have very generously proportioned bathrooms and living spaces, as well as the internal hanging garden. The spa, restaurants, pool etc are very nice and benefit from a modern aesthetic, but they're not quite in the same class as a high-end hotel such as the St Regis (or the Pen / MO, for that matter). Having said that, we stayed just after it opened and benefitted from some extremely generous soft-open rates, which were much better bang for buck - not sure whether the current rates hold up as well when compared to what you get from the other hotels.
(Sorry for the OT)