Check your UPS battery

My office is equipped with three uninterruptible power supplies. Each one can run its equipment load for 5-10 minutes in the event of a power failure. These little gizmos really come in handy when very brief power outages (just a few seconds, typically) occur. If the UPS is doing its job, you barely even notice, and you don’t have to restart any hardware or software.
Except when the battery backup itself doesn’t work. See, those UPS units run on batteries, which eventually fail and need to be replaced. It is much better to replace the battery before it goes dead than it is to discover that your UPS can’t perform its intended function.
Long story short, one of these batteries failed recently, and so to honor the occasion I’m going to replace the batteries in all three units. (They’re all roughly the same age, so if one fails, the others are likely to be close behind.)
These are Belkin F6C1000-TW-RK units. Although they list for $159, you can usually pick them up for under $120 (an Amazon.com partner has this model right now for $101.04). They’ve been very reliable through the years, and Belkin’s quality and support are excellent.
So who else has a UPS? Has it ever saved your bacon, or has it failed when you needed it?