The xi series was introduced in 1991* and included lenses and camera bodies. 'xi' included the traditional screw drive auto-focus interface (A-Mount) but added a motorized zoom function. Due to this, xi lenses are only compatible with post xi era cameras. The xi cameras included the 9xi, 7xi, 5xi, 3xi and 2xi. They follow the Minolta standard for naming, where 9 represents the highest grade, typically "pro" and as the numbers go down, the body generally has fewer features and lesser build quality. According to my research, there were 5 xi lenses released as well including; 28-80mm xi f4-5.6, 28-105mm xi f3.5-4.5, 35-200mm xi f4.5-5.6, 80-200mm xi f4.5-5.6, 100-300mm xi f4.5-5.6.
The main feature of the xi line was the motorized zoom. Instead of turning a zoom ring in the traditional manner, you turn the ring one direction or the other a few degrees of rotation and the motor would zoom the lens for you. If that sounds a bit awkward to you, you are not alone. Turning the zoom ring on a well made lens is a pleasure - the xi lenses were not. You do get used to it once you use it for a while however but you will never have the ultimate precision of the mechanical zoom ring either.
So why would Minolta implement such an odd idea? "From the Mind of Minolta" come innovation and in this case, body actuated zoom and other "zoom effects". One of these effects was "subject tracking" which would zoom automatically to attempt to keep a tracked subject the same viewfinder size while it moves closer or further away from the camera. Other effects would move the zoom during exposure to create an effect. etc.
xi in my experience: I have owned and used a 7xi body and was pleased with it. It does not live up to the 7 (non xi), but represents a good value. The 7xi can be used with standard a-mount lenses, so you are not limited to the xi range. I have also had the 100-300mm xi f4.5-5.6 and found it to be quite good. The build quality with the motor zoom was not quite what I expect from classic Minolta, but still better than most inexpensive modern lenses.
xi in my opinion: Very good value. xi was very unpopular and not well understood. Anything with the xi name carries the stigma despite being a good quality product. You do not have to use the odd xi features and if you are at all like me, motorized zooming is something you will get used to (get over) quickly.
If you are looking for bargain Minolta equipment, please consider the xi line - it is often overlooked. The 7xi and 9xi bodies are good items to look out for but compare the price against the preferred "real" 7 & 9.
All of the lenses get decent reviews but if you find a cheap 100-300mm xi, that is probably going to be the best value. There are already plenty of inexpensive non-xi lenses to cover the other focal length ranges.
* The best reference I can find seems to indicate that 1991 was the xi introduction. I'm not 100% sure that is correct.