So as an update after a four night paid stay here recently, and answering some of wantan's questions:

Platinum recognition: Very good, was upgraded a week in advance (I noticed via the app) to a 'river view suite', free breakfast (even though it is a resort) and 20% discount on food & beverage purchases (excluding room service, but including the nightly buffet). There were other perks (free, very slow internet, spa treatment discounts, 20% off laundry, 10% resort shop discount) and a very good welcome gift - we went for chocolate and had some massive creation that provided us with desert for a few days.

Location: Excellent, if you are in Mulu as a status member I can't see any reason not to stay at the hotel. The shuttle buses to the national park are very frequent (on demand) and the level of hotel is better than the national park offering, though that was cheaper (particularly food there was about 30% of the Marriott one). We were however staying at an off-peak week so we didn't struggle to get space, though it was fairly obvious that groups got priority onto the transport, but they tried to get you there asap. Leaving 30 minutes before your park tour was plenty of time and could mean arriving between 25 and 5 minutes early.

Pricing: You are highly likely to eat at the resort for breakfast and lunch, and it is priced for a jungle location (all food is air-freighted in), so pricey. We paid around 450MYR/night (USD 120), and then spent (pre-discount) 88 MYR per person for buffet, 160MYR for a house wine and 30MYR for a beer, slightly more for a cocktail).

Staff: They try hard, service isn't perfect but was reasonable (i.e. you needed to catch the staff's attention to order, but then they were prompt, but didn't always clear dishes as quickly as you may want). The general manager is very visible and clearly trying his best to instil international levels of service and attention.

Room: Modern, great balcony, lots of space, walk-in shower, big bath, well looked after. A minor annoyance was that the nice Samsung TV only had 6 channels and had screen sharing disabled, so I couldn't cast my tablet.

Internet: Worth highlighting that the park doesn't lie when it says internet is limited. There is a very slow internet in the lobby, though not in the rooms. It was very intermittent and couldn't be relied upon for more than occasional text messages. I did buy a SIM card, knowing of this, which was slightly better, but still fairly limited (SMS only for 2 days, then all of a sudden it offered '3G', but I had to keep reconnecting to keep connected). That was from Xpax / Celcom, the national provider. It was marginally better than the hotel internet and may be worthwhile if you really want the internet, however this is a place to go and relax, or the national park wifi was better, and only cost 5MYR/day.

Building: All wooden, raised on stilts, close to the river and jungle, not a typical Marriott

Pool: Perfectly suitable to relax in/around, but realistically you are going to spend most of your day on tours and in the national park

Tours: I noted that the Marriott also seemed to arrange tours into the park as well if you wanted to do a private trip, or with a few extras (e.g. free coffee, nibbles), though not much use for us since we booked via the national park months in advance

Otherwise I'd echo all the points from MrColdShower, certainly a memorable Marriott, and one to return to if I was ever in this area of Malaysia again without hesitation. My only minor niggle was that they declined to give me a late check-out for another 90 minutes, though as the hotel was apparently fully booked perhaps understandable. We did hear a complaint from a fellow traveller that he had to change rooms and ended up near a generator that disturbed his sleep, however clearly status guests were kept away from such issues!