I have flown from HKG and KUL via BKK and AMM to LHR in J many times. the prices are sometimes amazing compared to BA and CX. The seats are not quite as comfortable - they are flat but not completely horizontal.
At BKK you may have to leave the plane while they refuel. sometime you are held in a secure area and sometimes they let you free to duty free area. On one trip I took advantage of my BA gold card and raced to the BA/Qantas lounge where I had a shower and grabbed some food and newspapers.
Food on board is amazing. Instead of asking you to choose menus they bring the lot and you choose which items and how much of each you wish to have. they have onboard Nespresso coffee.
Generally service is good. Much better than BA who feel they are operating a military service but not quite the quality of far east carriers such as CX.
The lounge at AMM is very good with plenty of food and places to rest. They have a very nic rooms to rent in the lounge which are more like hotel rooms. The rest of the airport is a dump.
Transit in AMM is great if you want to take some time off to explore the place. You need at least a full day (or more likely one night) to explore Petra and onto Aqaba. Or you can visit the Roman city of Jerash in the north. stay in Amman and explore the city or travel 45 miles or so to Jerusalem (but the border crossing may take a few hours). In the old days could have travelled to Damascus in about 4 hours by shared taxi.