ID90's are 90% discount off of the full walk-up fare. They are mostly being replaced by ZED fares...similar concept but ZEDs are based on mileage not walk-up fare, so it's usually better for the traveller (you don't get hosed on a high-priced business route).

If you have to get to cousin Sue's wedding in sacramento, probably just JS or buy a ticket on SWA. But if you're going overseas for three weeks and have some flexibility in your schedule, ZEDs can save you a large chunk of change. Just monitor the loads in advance so you know where you stand.

ID90/ZED have lower priority than company employees who are non-reving.