I used the SQ lounge at AMS recently. It was pleasant, but nothing all that memorable. Some decent snacks, drinks, etc. Small room. No view of the airfield. I didn't check out the contract lounge that UA uses, so I have no point of comparison.

The SQ lounge (and all lounges) are upstairs, after passport control, but before carry-on luggage inspection. Each gate (or pair of gates) has its own scanners and game of 20 questions, so there was not much of a wait to get into the secure gate area. I'm a brisk walker, but the walk from the lounges to the UA gates is substantial, if this is a concern.

The SQ lounge attendant offered to alert us when our UA flight was boarding, but we actually left the lounge before she could notify us. (Had to hit duty free...)