Firstly, a big plus was that this trip was free due to Hotels.com rewards night! :) We're annual visitors to Marmaris, have stayed in many hotels in the area and the Cettia rates well - even if we'd paid! The Adults Only rule means this feels more relaxed; the décor is superb, staff friendly and efficient, jetty access nice and, if you like it, (not bothered myself) the evening entertainment good.
The Spa is well worth a visit (£60 for a 2hr sauna, mud scrub, foam bath and tanning massage is a good idea on the first day). The restaurant food is good with a Bar-B-Q option every meal time and there is always something available from Ice Cream to Burgers to Afternoon Tea. Another plus was my hotel nights were upgraded to all inclusive, although it's certainly worth getting out for some meals elsewhere.
Unfortunately, there were two frustrating aspects for me. The Wi-Fi is really poor unless and there's a limit to three connections. This could be remedied. Sadly, the other issue can't and that's the basic design of the building that has sea-facing balconies on the side for two of the three rooms on each floor. There's no opportunity to lay out on the balcony (too small anyway) and travelling with a disabled wife who needed to lie down in the room rather than the pool, this presented difficulties. There's a patio style window but the room still felt stuffy in the heat and it did have me casting envious eyes to adjacent hotels.
Recommended but may not suit everyone.