It's an incredibly cost-effective use of miles if you check the fare difference from discount coach to business class. Two minor comments:

1. There's also a 25,000-mile one-way upgrade that may come in handy if you want to fly one direction with a non-upgrading companion or if you can get an exit row (not bad on AA's Airbusses) one way but not the other.

2. It's confirmABLE at time of reservation, emphasis on ABLE, but won't necessarily be confirmED. If you reserve early you may be waitlisted until they're sure paying passengers won't come along, or perhaps until they're sure higher-priority upgraders won't. In my experience these upgrades have always cleared, but sometimes not until the day of the flight.

This past April (ORY-BOS) I lost my one-way certificate, took a coach boarding pass, got upgraded at the gate anyhow because coach was oversold, then found the certificate on the seat of my car at the Boston airport!