I used to live in Bangkok and was a Ruen Nuad regular for over 4 years. It used to be small and friendly. I just returned on a short visit to Bangkok and had a very unprofessional and un-relaxing experience. It now appears the place is too busy, and its systems can't cope. It no longer has what made it a great place. I see other reviews here confirm it's now a bit "hit and miss" which s such a shame.
I was about 45 mins into a 90 min massage when the woman from the front desk/reception burst into my private room telling my masseur he had a booking and dragged him away (note that they take the fee upfront so I agree, he had a booking, ME!!) There was no communication with me. She'd brought a replacement masseur who looked a bit confused and dumbfounded. I tried to give it a go for 10 mins but the whole relaxing vibe had been destroyed by the intrusion, the style was different, he didn't know what had been done or not done etc so I excused myself and left.
I've never had such an unprofessional experience at a spa in my travels. If the small, friendly local place is no longer small and friendly then you might as well go to a Healthland or similar - larger, but more professional than that.
If you expect your private room to be private and your massage to be uninterrupted give some other place a go.