They used to be glitchy, I've had okay success with them. The igc module (new igb) is a buggy nightmare on Linux with the kmod maintainers having no idea how to run a speed test with forwarding to also kernel panic... It's really bizarre.
Intel's consumer-oriented Ethernet has also been a disaster on the hardware side lately, with basically all of their 2.5GbE chips having serious bugs. At this point, it seems like Realtek is the sane choice for 2.5GbE, or Aquantia/Marvell for 5/10GbE.
