In practice, 10G-T today can be finicky
Not all that long ago I wrote an entry about why I think that 10G-T will be the dominant form of 10G Ethernet. While I still believe in the fundamental premise of that entry, since then I've also learned that 10G-T today can be kind of finicky in practice (regardless of what the theory says) and this can potentially make 10G-T deployments harder to do and to get reliable than SFP-based ones.
So far we've had two noteworthy incidents. In the most severe one a new firewall refused to recognize link on either 10G-T interface when they were plugged into existing 1G switches. We have no idea why and haven't been able to reproduce the problem; as far as we can tell everything should work. But it didn't. Our on the spot remediation was to switch out the 10G-T card for a dual-1G card and continue on.
(Our tests afterwards included putting the actual card that had the problem into another server of the exact same model and connecting it up to test switches of the exact same model; everything worked.)
A less severe recent issue was finding that one 10G-T cable either had never worked or had stopped working (it was on a pre-wired but uninstalled machine, so we can't be sure). This was an unexceptional short cable from a reputable supplier and apparently it still works if you seat both ends really firmly (which makes it unsuitable for machine room use, where cables may well get tugged out of that sort of thing). At one level I'm not hugely surprised by this; the reddit discussion of my previous entry had a bunch of people who commented that 10G-T could be quite sensitive to cabling issues. But it's still disconcerting to have it actually happen to us (and not with a long cable either).
To be clear, I don't regret our decision to go with 10G-T. Almost all of our 10G-T stuff is working and I don't think we could have afforded to do 10G at all if we'd had to use SFP+ modules. These teething problems are mild by comparison and I have no reason to think that they won't get better over time.
(But if you gave me buckets of money, well, I think that an all SFP+ solution is going to be more reliable today if you can afford it. And it clearly dissipates less power at the moment.)