I'm a United Premier member and do a lot of flying.
Customer service: on par with everyone else and better than Hawaiian (I'm doing Hawaii comparisons because you're from Hawaii).
Economy seats: Slightly roomier (just slightly) than Delta and Hawaiian. On par with Aloha. If you fly with a Premier member, your entire party can be upgraded to economy plus which gives you several more inches of leg room.
If you take the red eye from Honolulu to LAX (a lot of eastbound flights leave at night), they no longer have a meal service (they used to serve deli sandwiches in economy) - so bring your own food if you think you'll get hungry. The other day flights still serve meals. All flights have beverage service.
In my opinion, unless you NEED the extra space (because really, that's all that you'll be getting if you do the red eye), it's not worth the bucks to upgrade.