Hey kaborka. I thought I may be able to help ya out. I've actually used this forum for perhaps almost 5 years now, but never registered. I tend to read a lot but never post on forums, lol. Great hot deals here! ;-)
As far as the PowerChute software is concerned, it is not mandatory for basic functinality. Although I would recommend that you install it and at least play around with it for a while. Specifically for the options to control the over/undervoltage points where the unit switches to battery power. Also for the option to control the threshhold for line noise. I personally use the tightest settings across the board for the best power at a trade-off of wear'n'tear on the UPS (which is still minimal where I live). Now I'm not sure if these settings are stored in a non/volatile chip in the UPS or if they are sent via the USB cable at OS load. I would hope that the unit stores the voltage level settings instead of resending them every reboot, therefore allowing the one time use of the PowerChute software. I havn't tested on that point. I hate running extra services/apps on boot. I'm currently running 4 copies of XP-Pro on one harddrive using partition hiding and a boot manager. Each OS install is custom tailored to specific purpoes on the points of performance, functinality, and security. On this old Athlon XP running at 1.65ghz, XP-Pro (set for offline gaming) boots in 11 seconds. I currently use niether the PowerChute software nor the UPS service. (Although I have for years in the past.) As a matter-of-fact, I'm currently not even running the USB cable as I dont require auto-shutdown. I don't even have USB turn'd on in the BIOS, lol.
As far as brown-out protection goes, you can set them to switch over to battery at a fairly high voltage setting but the Back-UPS ES line does NOT come with AVR (automatic-voltage-regulating). So there's no circuitry to compensate for small sags w/o the battery. As a matter-of-fact, as far as sags are concerned, the unit is either on battery, or doing nothing.
Also, the model on sale was actually the BE500U (6 total outlets) and not the mis-advertised BE500R (8 outlets, newer revision). I'm not sure if the BE500U has been or will be discontinued but I imagine that it has been since it is no longer listed at APC.com. I've had one for about 4 years now, and this computer is running on it at this very moment. It still runs great and the battery is still in good condition. I probably average 20-30 "quick-clicks" to battery and back each year with perhaps 5 total brownouts+ in the 4 years I've owned this and the computer was actually running. Works great every time. I bought 3 more Friday morning.

Relatively speaking, the deal was barely listed online. At 11AM my OD had 8 remaining with over 20 at stores withen a 10 mile radius of me. If I had more liquidated capitol I would have probablly bought 20+ that morning to sell to customers with system-builds. Great deal for your customers.
BTW, I've used APC UPS's from 3 different product lines and currently own 7 APC products. Personally I think the Back-UPS ES is the best line from APC (under ~$100+) unless you went to the very expensive Smart-UPS line. (Which kicks A but is major overkill for any sub-$3000 system) Some of the units from slightly higher families (including ones with AVR) have often caused horrid transformer harmonics when on battery. (The first stage of power circuitry inside monitors would often produce a LOAD of noise for instance). Some UPS's batteries contacts are only spring clips and if jostled they many break contact for a second... (bad) The ES line uses wire terminals with a very snug fit and doesn't require the user to twist and stretch the crap out of the internal battery cabling like some other models.
Anyway, enjoy my encyclopedia posting! I type 90 WPM and tend to ramble... LOL now you see one of many reasons I often don't post on forums.
----> Since it is 4:30PM PST on Saturday and allisolm mentioned it is still good today, you may want to check out your local OD if you havn't already. At this price, it's a steal!