Quite good and easy to use. Theoretically, it has a range of 100 meters in an open space. I tested it at a distance of 60 meters with two relatively thick walls in between, and it had a signal line. So its features are indeed as advertised. It has 4 settings for sound sensitivity and lights up with 3 indicators showing the sound intensity. You can also remotely adjust the parent unit, turn on the light, play a song, or even speak (I wonder if there is a baby who sleeps while you talk to them through a walkie talkie, but anyway). I have had it for about a month now, and it seems to be worth its money so far. It connects easily, is user-friendly, has plenty of functions and settings, and works with or without batteries (directly plugged into the socket). The only annoying thing is that it makes a sound when you turn off the parent unit (fortunately not when you turn it on). So if you are, for example, outside or on the balcony and then enter the house and close the device, you will hear a sound. Therefore, you have to do it before you enter. Not a big deal, but it really seems useless as a feature of the device.

For those wondering about the differences between the other models, here is a summary:

Philips Avent SCD731 has the same features as the SCD580.
Philips Avent SCD721 has the same features as the SCD570.
Philips Avent SCD711 has the same features as the SCD560.

The only thing that changes in these is the appearance and the fact that the SCD5XX series is mainly available in the US/UK, while in Greece, Philips only offers the 7XX series. The extras in the name with / (e.g., SCD731/52 and SCD731/26) are simply differences in batch and date. Otherwise, the functions are exactly the same, you can check it out. Don't fall for the American/English model, which is usually more expensive, thinking that it will do more things or be better.

In summary, the differences between the models from the simplest to the most complex are:

SCD506: The simplest model. It has a light, music, and the useless ECO MAX function (which basically means that the connection is only made when the baby cries. This means less radiation but also a longer delay to hear the cry, and I have doubts about how easily and quickly it will connect if you are far away).
SCD711/SCD560: The difference from the SCD506 is that this one has a temperature indicator and alarm (maximum/minimum). Theoretically, it has a range of 10 meters more in an open space than the SDC506 (the SCD506 says 300ft, the others say 330ft).
SCD721/SCD570: The difference from the SCD711/SCD560 is that this one has the option to notify you with vibration for temperature changes (while the SCD711/SCD560 only has sound). It also has a base for charging the parent unit, which, in my opinion, is useless because it just takes up space and instead of connecting the device to the charging cable, now you have a base that needs to be connected with a cable and you have to place the device on it to charge.
SCD731/SCD580: The difference from the SCD721/SCD570 is that this one has an extra option to project various patterns on the ceiling.

I hope I helped and saved you some money