This review is for the 10GTek X550-T2 (PCIe 3.0) card, not the X540-T2 (PCIe 2.1), though I suspect that the same performance would be seen with the X540-T2.

This card works great for me!

I bought 3 of these NICs and a Netgear 10gbps switch (XS708T) as I was tired of my 4x1gbps (Intel Pro 1000 PT NICs) Windows 10 auto aggregated links dropping to 1gbps due to MS dropping support for the automatic aggregation that they accidentally added into Win10 a few years ago. MS pulled this very nice (accidental) support and I was back to 1gbps without doing full switch configured LACP aggregation. I got really tired of backing up 45TB on a 1gbps connection. So I threw $ at the problem and hope to be good until 40gbps, someday.

These NICs works well for me via single port (9.9gbps) or dual port with aggregation (> 10gbps).
I am running this on Windows 10 Pro and Workstation with the standard (Windows Update) drivers that were automatically installed.

Testing
I have tested with iPerf3 and even with single, a few or many threads, it is very hard to saturate the connection and show > 5gbps. So for me, iPerf3 just won't do.
I now test by performing long running SSD to SSD tests through a Netgear 10gbps router which shows me hitting 10gbps (and greater with dual ports and aggregation).

Real World Feel for my daily workflow
I have also tested my heavy Visual Studio work flow with my solutions and projects running on SSD on an older 7700k system and the connection seems very snappy, barely noticeable from my main local storage which is 3x Win10 stripped NVMe (Samsun 970 Evo Plus) 1TB drives, where I get 7GB/s sequential block performance and snappy small random block performance. I suspect that if I use this NIC on a mid-high-end NAS with 10gbit NIC that I will be satisfied with the perf of my workload (lots of small files). I will update this review if I am able to test that in the future.

Windows 10 auto link aggregation (just plug multiple single-machine Cat5/6 cables to your switch)
As of recently (this NIC or Intel Pro 1000 PT) I _DO_NOT_ see automatic aggregation with Windows 10 Pro, but I _DO_ see automatic Win 10 aggregation (this NIC or Intel Pro 1000 PT) after upgrading my Windows 10 Pro servers to Windows 10 Workstation (I don't use Windows Server). I see spill-over behavior where when the one port is saturated (10gbps) then the 2nd port starts taking on any addition load (I have only gone to about 13gbps) . The only reason I upgraded to "Workstation" is that I have plenty of OS keys from MSDN, so why not try it - it worked. Not sure if you would get the same same result - so don't assume.

I will keep using these NICs unless I see a reason to not to (which I would add to this review).

Wanted high bandwidth connectivity from my older homebrew FreeBSD router/firewall/server ( don't judge me... reasons. ) to my new Ubiquiti Switch Pro 24 PoE.

Popped this in, along with a pair of https://www.amazon.com/gp/product/B0753BHLQY/ SFP modules and connected by https://www.amazon.com/gp/product/B0897CNRR5/ OM3 fiber.

Fired up the PC, configured the NIC, and was off and running.... zero drama!

It has bee up and running for 4-5 months now, zero issues.

Happy. Asumming PCI slot availability, I would not hesitate to use this configuration in simlar future installs.

Identified and enabled by operating system. Works with Cable Matters 10GBASE-SR SFP+ to LC Multi Mode 10G Fiber Transceiver. 10 GbE.

Replaced a 'name brand' that I bought new but would run a few days then start generating errors. Thought I would try this until a replacement arrived. Worked so well with 0 errors that I have not bothered to replace it. Going on 7+ months with no issues.

I was searching high and low for reliable 10gbe for a TrueNAS system. I put these cards in a TrueNAS and Windows machine for a direct attach setup and it's working flawlessly so far. Cheaper and easier than trying to hunt down old Intel X series or Chelsio cards. The use of off the shelf Intel drivers makes this card really easy to set up.