I have an AP21/22 on my boat and it is absolutely perfect. I upgraded the J300X, AP21 and AP22 all with the latest firmware (EPROMs) and everything functions flawlessly. These systems were only replaced by the AP24 in 2007, so they are not really all that old. I also upgraded the system with an RC25 rate compass, which is much more precise than the standard fluxgate compass. I even installed a FU50 follow-up steering lever or jogstick at the upper helm to provide steering capability. The system is connected to a Garmin GPSMAP 546S. In nav mode it will maintain about +-30' cross-track accuracy, which is sufficient for all but the narrowest of channels in my area. My boat has hydraulic steering with 5 turns lock to lock, and I can assure you the Simrad has made a significant difference in operating the boat as I am almost always solo. To windward, it steers the boat like it is on rails, and in beam or following seas, 'Lincoln' steers a better course than I do. I call it 'Lincoln' because it freed the slave . . . . .
As long as your nav equipment still supports NMEA0183, you will be able to use this autopilot for quite some time. If, however, your systems are more oriented toward NMEA2000, or you are looking for direct integration with late generation Simrad electronics, you will probably be better served with a newer unit. The AP24 and later systems have some great new features $$$$$$$
There are still plenty of parts available for these systems and Simrad technical support is second to none.
Let me know if I can help you with any more info on this system as I am pretty familiar with it by now.