The Crowne Plaza Hotel adjoins Changi Airport and is definitely the pleasantest place to spend a few hours or a night when catching connecting flights in Singapore. You do have to check out through immigration and customs with your luggage but you can keep your luggage trolley with you until you reach reception.
The hotel is modern with a lot of black marble and dim lighting – either relaxing and cool or a bit too dark depending on your personal feelings (we were one in favour and one against). Check in was swift and friendly and our room well lit and comfortable. We had a swim and relaxed but did not eat in the hotel so I cannot comment on the food.
In summary, it is a bit expensive for a few hours but much more enjoyable than waiting around at the airport. Changi does offer a free coach trip around Singapore if you are in transit for several hours during the day – we have done this in the past and it was excellent but we were tired and felt like relaxing this time.