Excellent choice for a server with low power consumption and simultaneously capable performance. It draws 11.8 W at idle and 24.5 W peak (measured with a Tapo WiFi plug), and I use it with 2x 4GB DDR4 SO-DIMM, with a Sandisk 256 GB SATA-3 SSD, running Proxmox VE 7.2 with ZFS-on-root, 3 LXC containers (pihole, gitea, radicale) also as a Jellyfin streaming server and Wireguard beacon. The only downside is its price, which is relatively high. Probably the lower model with the J4125 would do just as good a job.

EDIT: One year later, it remains an excellent purchase, and it has been running 24/7 since then. It is supported by the intel-media-va-driver-non-free package from Debian and thus does hardware accelerated transcoding on Jellyfin with impressive speed and low power consumption. Note that it supports up to 2x32 GB (!) DDR4 SO-DIMM (see https://www.mini-itx.com/~J5040-ITX), making it currently a much better choice than the new motherboards with the N100/N200/N300 which only support single-channel and usually have only 2 SATA ports.

EDIT2: The little server with the ASRock J5040-ITX has been running 24/7 with 4-5 reboots since I set it up. It serves everything seamlessly, with a consumption of about 25 W at 40% CPU load, with 2x 8GB DDR4 SO-DIMM 2133 CL17, 2x 500 GB SATA HDD 2.5" in btrfs RAID1 (with Proxmox VE 8.3 + Proxmox Backup Server) and a 2 TB SATA HDD 3.5". Truly one of the best motherboards on the market. A bit pricey for its performance, but the overall cost of acquisition and operation over three years is very satisfactory. Five stars, easily.

The best tool for HTPC and more. Without temperature issues, it plays everything in 4k @60Hz sound and image, perfectly compatible with the Windows 11 update. The only downside (for those who consider it a downside) is the lack of HDR.

FANTASTIC motherboard with built-in fanless CPU which also does hardware decoding of HEVC 4K files! I have it in an HTPC and it plays 40GB+ files without any issues! Almost unbelievable.