The reason you didn't see the SQ lounge on the way to the TG gate is that it's in a different area of T1. I think for TG and LH departures you turn right after the dutyfree shops, while for SQ (and CX) departures you turn left. As noted before, you will need to go through another security check (stricter this time, requiring you to take off your belt) before you gain access to that area.

The food selection is limited (I think they really should re-think the spaghetti with tomato sauce), but the range of beverages is acceptable. You do get views of the runways, and unlike the CX lounge, you won't be required to step out of the lounge to use the toilets. First and Raffles Class lounges have separate entrances (again unlike the CX lounge, which requires you to walk through the Business Class lounge to enter the First Class lounge). I found the computers (4 terminals in total) to be rather slow.