I had bought an Ultracell 100AH battery in the summer of 2014 for a small photovoltaic system, daily use for lighting and a few hours for a small refrigerator. I bought another one in the summer of 2016 because I didn't have the power reserve I wanted with just one, and I connected them in parallel, which I later learned was not recommended.
After 5 years, the first battery significantly depleted, from the theoretical 1000A it originally had, it now stores around 230A, while the second battery, after 3 years, still has 550A, measured with a corresponding instrument. For the usage I have, I would say they are very good, still performing well.
Two months ago (September 2019), I thought about buying a new battery to help the system, and since Ultracell batteries had become quite expensive, I decided to go for a more economical option.
So, I bought another brand, a SAOOER 120AH GEL battery, which turned out to be disastrous, it was basically useless, my system couldn't even last a night because there was a lack of power in the morning hours. I had to buy a measuring instrument and I saw that my new battery only had 250A power reserve, while it should have had over 1000A. Fortunately, the store where I bought it accepted the return, but I lost 15€ on shipping. Since this experience was a "lesson" for me, I decided to buy the reliable Ultracell 100AH battery again to have peace of mind.
* If Skroutz opens a chapter for SAOOER at some point, I will write more.

How buying a battery became an "adventure"...
After learning my lesson with SAOOER, in early November, I bought an Ultracell 100AH from an online store for around 200€ plus shipping.
As soon as I received it (while recording a video), I opened the box to measure it before connecting it to my system. There was the first negative sign, the terminals of the new, sealed battery had white salts, not a good sign for a battery. I measured the battery and unfortunately, the bad image was confirmed, the power reserve was only 595A, while the datasheet states Max Discharge Current 1000A (5s).
Nevertheless, I installed it in the system to go through a few charging cycles in case it would recover, but the maximum it reached was 605A. I contacted the store, sent them the visual material, and after a few days, they replied that they informed the supplier who told them to send the battery back.
Several days passed and I was waiting to see what would happen, eventually, the store informed me that the supplier acknowledged that the battery was indeed faulty and they offered to replace it with another one. I asked the store to check the new battery before sending it to me, so I wouldn't have the same problem, but the answer I received was that they didn't have that capability...!
If possible, what I asked for was to send a tested battery, and they said no, I never expected this from a reputable company like Ultracell, it ruined the good image I had of this brand, and along with it, its reliability was lost.

After the trouble with Ultracell, the store offered me a refund or to find another battery for me, and I thought that since I already lost money on shipping, I would give it a chance to find a solution. They suggested another brand, the Winner Hercules model, which the supplier had the capability to check for proper functioning. All this process took the whole of December, I needed energy and I didn't have it and I kept waiting until finally New Year's Eve came and the Winner arrived, a heavy 44 kg battery.
I opened the box (always recording the process) and took the first measurement, but the smile froze again, the power, the energy of the battery was about 450A and that's when I started to lose my mind. Three months passed, with lost money, countless messages and phone calls, and still I didn't have a decent battery.
I installed it in the system but the reality is what the device showed, the battery barely lasted through the night, sometimes it couldn't even make it until dawn. After New Year's, I contacted the store, told them I was packing it up without a second thought, sending it back and asking for my money.
*If Skroutz ever opens a chapter for Winner, I will write more.

15/1/2020
I went back to the effort of finding an Ultracell 100Ah battery in a store that would give me the opportunity to measure it before receiving it, it was a reasonable request after everything that had happened to me. No store accepted such a thing, even when I contacted the main supplier of Ultracell and asked to be allowed to measure it, he also refused. They all told me the same old story "...our batteries, sir, are tested and have no problem..."
After several fruitless attempts, I gave up any idea of buying Ultracell because after what I wrote to you, it seemed completely unreliable to me and this brand is finished for me.

The initial 5 STARS for this specific product are now 2, and that's only for the sake of the past, if I were to rate based solely on the last purchase, I would give it 1 STAR.
NOTE: I have all the relevant evidence for everything I say, emails, invoices, photos, videos.

28/1/2020
After a lot of searching, I finally found the solution here:
https://oknow.gr/hlektroniko-katastima/fotovoltaika/mpataries-fotovoltaikon/solar-batery-220ah-sinergy-italy-leptomeries.html
https://oknow.gr/hlektroniko-katastima/fotovoltaika/mpataries-fotovoltaikon/solar-batery-250αη-250ah-12v-solar-deep-cycle-ανοιχτου-τυπου-leptomeries.html

As other people have mentioned, it is a proposition with a low price. I have 2 pieces in UPS a little more than time and everything is OK (so far). The only negative is that compared to some other slightly more expensive ones, they have a shorter duration (I saw this from the first day I tested)... however, this time personally is enough for me (and that's why they were purchased) to power off my PC and TV that I have on top, I hope to get 3 years. If they manage to reach 4/5, it is fair, otherwise I will come back to change the rating because there is a wide range of options.

Update 5/7/20 and the problems started, capacity problems with the PC while it is on the UPS, it does not turn on correctly, one of the two has died (I have not investigated yet due to limited free time) but I will measure values ... so I lower the rating to average because they have a maximum of 2 years of life and even in good conditions! The previous ones lasted 3 years and even in worse conditions (Island) and not in Athens where these are.

Conclusion: Cheap but not so reliable. 2.5/5

It is a relatively good to medium battery at a low price. If it lasts you two years, it's great. However, don't expect it to last longer. Generally, most batteries at this price range will last you about that long, unless you come across a defective one and it dies earlier. It is good to unplug the UPS from the power once every one and a half months so that the battery can work properly and last a little longer. I use it in four UPS devices. In one of them, with factory batteries, it lasted for about two and a half hours. With these batteries, it lasts for about one and a half hours. In the other three UPS devices that I have, it lasts for about one hour when the factory batteries lasted longer. I also have a small 6v 4.5 ah battery in a security light. It didn't last. Over time, it died. Even though I cycled it correctly, by unplugging it from the power. It is a relatively good to medium battery at a low price.

Also, I have the battery in an alarm system. So far, it has been working perfectly for three years. And in the two sirens that I have, I use the same brand but with a smaller capacity in amps, and they have been working for three years without the alarm indicating that the battery needs to be changed. I didn't expect them to last that long.

Maybe in the UPS devices, due to the temperature that develops, they last less.