Applevision monitors had a good and bad reputation. I believe that the 850 had a better reputation. Usually Apple 20" monitors lasted a long time in what would be considered hard use, like on all of the time. For the price you have specified, that is a bargain, but don't be hurt if it up and fails after 2 or 3 years. You got an inexpensive 20" monitor and I would be happy with that lifespan.
Also watch the connectors. I believe that it has a traditional DB 15 connector on it and will be incompatible with the newer HDB 15 connectors on new Apple video cards. You will need to dig up the adapter that was provided with your B&W G3/ or G4.