Yes, to USA savings can be quite substantial, especially including the domestic flights, if booked together. This also goes for booking on NH, with UA domestic, and I had once they charge like $5 (for the tax) per domestic segment.
As for service / food, learn to expect nothing, and you will appreciate their service.
SFO - HNL considered domestic flight, so expect domestic flight service (therefore no free headphones). Just bring your own headphones.
SIN - HKG also downgraded to like domestic flight status for economy, think no free alcohol too.
I believe SIN - HKG is by their 747, which do not have personal TVs in economy too.
Bring own entertainment device plus headphones. Some aircrafts now have wifi streaming, so you can stream to your own device.
Lousy / no food? I bring my own cup noodles and snacks. In fact, other than when I take SQ, I will bring cup noodles and snacks, TG / NH / UA / BR all do not provide other snacks/food other than the pre-meal snacks and standard meals. This is especially for long or overnight flights where they assume you will sleep and might not be hungry and you do not sleep and get hungry faster (or you missed the meal service if you sleep).
I'd avoid their 747s, if possible, since it is the oldest in the fleet and no personal TV in economy. BusinessFirst / GlobalFirst will be fine.