First, the battery is great, and works just fine.
Second, the seller has been responsive (same day, late afternoon), and even willing to replace the product if bad. But I didn't need to replace the product -- I just needed to understand that there is more to installing the new battery than plugging it in.
Since I haven't seen anyone else mention this on the other reviews, and some of the problems reported would likely be CAUSED by this, I felt I had to report my experience.
I read some negative reviews on this product, and other similar products by the same vendor, that reflected my initial experience. I bought the APC Back-UPS XS 1500 Replacement Battery Set anyway because it was less than half the price of the name brand. It turns out that, at least in my case, those negative reviews would have been unfair categorizations -- and some of those writers may have been a bit too quick to judge.
If you connect up the new battery, fire up the UPS, and expect it to work correctly, you will be in for a surprise. You MUST update the "REPLACE BATTERY DATE" for the unit by using the PowerChute Software from APC before it will work correctly. This is especially true if your old battery was completely or almost completely dead.
The APC Back-UPS XS 1500 is a sophisticated battery system. Like most systems, it needs some info to give accurate predictions -- for instance, it needs history. It builds that history over time -- starting from when you set the "REPLACE BATTERY DATE."
I am a Mac user, and never bothered with installing the software -- in fact, no current Mac software exists for it. So it was a pain for me to do this -- I had to borrow a PC to update the "REPLACE BATTERY DATE."
As soon as I updated the "REPLACE BATTERY DATE" the unit immediately began working correctly. It went from showing no battery charge and 1 minute of run-time (under a 7% load), to showing an accurate charge (70% or so), and over an hour of run time. I tested it further by running it on battery, and it reports accurate run time, and lives up to its report.
So, do yourself a favor, save some cash by buying this battery, make sure you have the latest PowerChute Software installed and update the "REPLACE BATTERY DATE." You find this under the Battery self-test panel in the software. This CANNOT be done on the UPS itself (or, at least it is not documented anywhere).