Fully supports all 2011-3 CPU's from what I've seen, you can double check on their compatibility list though.
Narrow-ILM just means you have to watch what cooler you buy.
As for it being a dead socket, I agree with EarthDog.
It's highly unlikely that Skylake-E will be on X99 and I doubt that Broadwell-E will be worth mentioning.
I don't think that X99 is dead as usually enthusiast line of chipsets has longer life than lower series ( up to 5 years ). For every X series chipset were 2 CPU series and in all cases next generation was faster and had additional features. What we got for 1150 ? Z87 and Z97 which were almost the same and Broadwell which on gamer/enthusiast market is total fail.
Haswell-E is totally different than 1150 Haswell. The same Broadwell-E will be totally different than 1150 Broadwell. Maybe Skylake-E will be also on 2011-3 and there is high chance on that as in earlier roadmap Skylake-E was described as next series for X99 boards while there was no Broadwell-E. Later Intel has changed all and Broadwell-E replaced Skylake-E ( maybe only naming but hard to say what we really get ).
Broadwell for 1150 is supporting DDR3 only and is like upgrade for Haswell with better IGP and in lower process while Haswell-E has already DDR4 support and some more , not to mention it doesn't have IGP. Skylake-E probably won't require new socket ( if Intel won't change anything and they are good in that ).
I don't think that Z170 will support more than 4 cores while X99 is already supporting up to 18 cores ( soon should be more as 20+ core Xeons were announced ). Also memory support is better on X99. I mean it works with more RAM and actually max memory clock at 16GB+ RAM is about the same as on Z170 ( maybe Z170 will improve but so far I doubt ). You can push DDR4 to 3800+ on Z170 but only with 2x4GB modules and on most boards it won't work above ~3400 anyway.
Regarding motherboards. Good Z170 board costs about as much as average, but still good for overclocking X99 board. Most features on all X99 boards are the same ( excluding some, the highest and most expensive series ). Of course you can get cheaper Z170 mobo but who will decide on cheap board for i7 6700K ?
Every X99 mobo is actually high end. Not all Z170 boards are really well designed. Even in Maximus Hero ( which I own ) ASUS was cutting costs and it's one of the highest ASUS Z170 boards available. In most brands Z170 boards have similar quality but 20% higher price than Z87/97. All improvements are mainly on paper while end user will barely see them.
Regarding smaller form factors. X99 ITX boards are really expensive and have some limitations like 2 channel memory but you can get micro ATX ASRock in pretty good price with all features ( except less pcie slots ) of full ATX board. For Z170 you will find cheaper standard series motherboards but everything from higher series will cost about as much as X99 ( I mean boards like Maximus Impact ).
At the end I would say that Z170 is good option if you can live with 4 core 6600K or lower CPU but if you want higher performance then adding these $100-150 to 6 core X99 platform is better idea.
Depending on Your upgrade path for other hardware, Z170 will support (via chipsets) more PCIe lanes which could afford SLI or CF along with PCIe SSDs (plural) or M.2 in RAID. And the MB folks are adding chips for C-type USB connectors as well as support for USB 3.1 v1 and v2. The development of more PCIe devices should follow the availability of the lanes. Toss in the PCM drives that will be coming out next year, which may become a defacto card for the future, and the lanes for traffic will be about like those for the freeway -> build it and they will come.
There are barely any devices for USB 3.1 and barely any can reach USB 3.0 bandwidth not to mention anything higher.
PCIe lanes are more like a theory. You can use as many graphics cards as on Z87/97. The only difference is that you can use one more M.2/PCIe SSD but who will actually make RAID on 2x M.2 cards ( if mobo will support it and I don't think you will find it in cheap boards ).
There are still no SATA Express drives and next connectors seem useless for 99% users. When all that will be on the market then we will see new chipset or new connector type.
Regarding new memory type ( XPoint ). This is only in plans for future. Releasing it for mass market will take next 3 years+. Micron was working on it 2 years ago and since then we only saw news that Intel and Micron are still working on that.
Ty - You make a good point... after thinking a bit about it, Broadwell-E, I believe, will be out on X99 socket 2011-3. That said, there wasn't any(?) IPC performance over Haswell-E so, in the end, I wouldn't consider that an upgrade. Skylake-E will have its own chipset/socket(?).
Broadwell gave up to ~10% improvement over Haswell at the same clock but base clock is generally lower and there is low max OC so in reviews comparing both CPUs ( like 5775C vs 4770K or 4790K ) at stock you can't see that difference. Still it doesn't matter much as barely anyone will buy Broadwell for desktop.
) - EDIT: Motherboard cost is a bit higher on X99... With a quick search, the diff is at least $50, a lot more if you want SFF
