I have some older Liebert UPS's with VRLA batteries (gell cells), but they don't last long and never seem adequately charged when I need them to work. I experienced a short duration power failure of around 10-15 minutes, just a day after I installed this new lithium ion unit, and my computer and network gear stayed up and running perfectly. I'll never go back to lead-acid batteries again! Another nice feature, is the lithium unit is noticeably lighter.

I have got this one as a replacement for an APC 1500.
Way much lighter to carry, it was easy to install, and it held longer than the old ni-cad battery. The sales person that I dealt with was very pleasant.

Picked this up to slip under the entertainment setup, which also has a server system in the case. The rack mount version works great for this, fitting into what used to be a VCR/DVD wide slot to support an old TV. After reading the reviews, I was expecting to need to install their software to do battery state detection for the system, or get an SNMP card for it, but it was auto-recognized by even old MacOS versions. It is silent except while charging or running on battery, and even then the fans don't ramp up much unless you push it towards the maximum load. I've been impressed by the extended run time this provides, especially compared to my old APC BackUPS 1500 with an additional external battery pack. This is more efficient, lasts longer on a charge, doesn't have as much overhead when keeping watts low, and thanks to the lithium tech, it should last virtually forever since it'll be in reasonable temperatures and not used extremely often. I just wish I could set it to only charge to 90% and not discharge fully either to make the lifespan of the battery even longer. Awesome and well thought out unit. Easy to pull the battery for a swap, which is a fairly standard LiFePo pack so replacements won't be a problem in the future, and I love the rotatable display feature. It is just a small attention to detail that makes everything else shine. So far I love it, though I've yet to install the software management for it on Linux to try and tweak with any further settings. I generally have good power, so this doesn't need to do any conditioning and kicking in for that, so while I see other reviews mention it has a wider allowable power range, I don't have a way to test that, nor am I worried about it.

Cons: There is really only 1, but I'll mention 3 for people considering the rack mount/tower one vs the mini tower. The first is the only real con...the fan is weird and pulses up and then down, every short bit, while charging. Reading the firmware updates, this MAY be fixed with an update, but I haven't done that yet. The second is that I wish it showed watts for output, rather than amps. This is a simple calculation, but I find watts more helpful to see when talking about system power draw and the like. The last is just something people need to be aware of. This thing is LONG compared to the mini tower. I measured and it fits for me, but make sure you do too if you're not putting it in a standard size server rack mount location. It would work well as a monitor stand...the old style CRT big bulky ones...but would take up way too much space to put on top of a desk. In the tower mode though, it is quite thin thanks to the standard rack mount sizing, so you could stand it up under/behind a desk quite easily. It comes with both rails for mounting in a rack and some feet for using it as a standing tower. The bonus con is that while the non-lithium series has external battery packs to add capacity, this doesn't...but for any reasonably normal use you won't have a capacity issue with this, and you could get a higher output capable version to pick up more capacity in the process if you really needed it and were set on these.

TLDR: Would definitely buy again at this point if I needed another, so long as the price is reasonable at that time.

I just got this hooked up so I cannot comment on longevity, but the Lithium battery should offer a much longer lifespan.

I mainly wanted to drop this review in response to another user trying to work with NUT. I had an APC UPS before replacing it with this far superior UPS and was using apcupsd. Without skipping a beat it just worked. Voltage reporting is missing, but load, time left and other important stats work just fine and automatic shutdown works without issue. I have not dug in further, but I assume it is just part of the USB HID spec.

If you have Linux and want to use an OSS solution do not shy away from the PSI5-1500RT120LI.

$$$, Quiet, Have to register/log into their website to get the monitoring software.
Used once already!
It did shut down the pc and was off when I got home, like I had it set to do. With power again, the fan did run while recharging the Li-Ions.