I haven't stayed there, but I had a look at it last August when I was hunting around for a serviced apartment. Its a good ten minute walk from the Chidlom Skytrain station although the sales manager who showed me the apartments said that there was a shortcut through the back of the hotel that made it a five minute walk to the Rajadamri Skytrain station. Its located on Soi Langsuan, which has several very nice restaurants, and taxis are easy to get...the doorman can hail one for you.
I didn't see the 3 bedroom suite, but the 2 bedroom ones were very spacious. Furniture had the sharp-edged look, if you know what I mean, and it came with a large flat screen tv, dvd player, microwave, washing machine/dryer. The pool on the top floor (25?) is awesome....very spa-like atmosphere there, although you can feel the area is small.
When I looked at it last year, it was brand new - only half the floors were ready and I was told several of the facilities would be completed "by the end of 2003". Even given Thai time, I think it should be fully functional by now
IIRC, you don't get stay credit, but you get 3 Marriot points per dollar.
I haven't stayed at the Marriott Riverside, but I'm biased towards staying in an apartment/suite if I was travelling in a group of 5 who were close friends or family.