I stayed at the Siam Kempinski for business for a few nights, and thought it was quite nice for a business stay. The hotel is fantastic, the location is right behind the Siam Paragon so you're close to the action but it's still quiet, fantastic room, a bed the size of a small apartment, and a good view of the pool, which looked great but didn't get a chance to try. Breakfast was included in the rate and was quite nice. I'm rating this 5/5 because it is really great, but there were a few odd things. The lighting system is badly designed in the rooms, there's no light just for the bathroom, the master switch by the bed turns off all the lights except for the bathroom lights (?), and there are a few little lights in the room that are practically impossible to turn off. I couldn't figure it out and neither could the others that stayed with me. There is a speaker in the bathroom so you can listen to the TV while you're in there (and watch it too, there's a glass wall separating the bathroom from the bedroom), but the speaker didn't work and the volume control did nothing. It's a new hotel, there's still a few bugs to work out.
You can pick up a cab at the hotel, or just walk across the street to the Siam Paragon and get on the BTS from there, just a few minutes walk through the mall. I actually took the airport train in to town, took the BTS two stops to Siam and walked to the hotel, took about one hour total. Not bad considering a taxi during rush hour would have taken a lot longer and cost way more. Recommend that way of getting in unless you're coming in late at night.