Good morning,
I'm writing this from Bangkok, where I recently stayed 2 times at the Double Tree, once at the Hilton next door (both hotels are indeed connected) and 2 times at the Millenium.
Just to report on this property, as a Diamond i was "kind of" upgraded in october and november, although that means a higher floor

Breakfast is nice and huge (it's the main one), no lounge access but a very nice 2 hours free drinks by the pool ! (if you want a lounge, book at the Hilton SUkhumvit, where I also scored a nice suite upgrade). The staff there is very nice.
Also no problems to check in early twice... + a late check out on the 2nd stay.
Overall a very nice property with great rates on a regular basis, and I like the fact that although it's not a "instagramable BKK infinity pool", you can actually swim there.
Have a nice day !
Sophie