I am using a XP941 512GB so I am able to put the OS, Games and Programs that I would use and still have room.  Just make sure you have Windows 8 at hand as its much easier to get the M.2 to work if you were to go with the XP941.

the m.2 is going to cause some confusion for quite some time on all mainboards supporting it.
luckily there guys here who already have experience with it so you get first hand info.
it would be also very nice to have some more m.2 models with small capacity just for the OS (60GB or 80GB).

WarGasimXDi
What exdactly is the difference between the m.2 ssd and lets say the standard SSD samsung 850 evo? I was thinking on trying out this m.2 but really dont know much about it

PCI-E Lanes vs. SATA Controller.  PCI-E Lanes bandwidth is a powerhouse compared to the SATA Controller.  SATA is limited.  Hence why you see around 500 GB/s on regular SATA SSDs.
 
Here's a taste of my XP941 512GB......
 

 
Also, since the X99 Classified is only M.2 PCI-E 2.0 x4 while some other competitors actually implemented 3.0 and don't know why EVGA never went that route, you can use any M.2 but make sure its M.2 PCI-E 2.0 x4 or you're paying more for a 3.0 possibly since it won't even run on those advertised max speeds.  I use the fastest one atm, which is the XP941 512GB since its M.2 PCI-E 2.0 x4.  The SM951 will be slightly faster @ 2.0 but might cost more since it caters for the PCI-E 3.0 x4 users as their advertised max speeds are intended for.
 
 

  GTXJB, (Sorry OP) Not to derail thread but..............Have you tried the Samsung Magician software on your XP941? I'm just curious if it will work on it. My reason for asking is:
 
This is my 840 EVO 250gb SSD via SATA connection with "Over Provisioning" enabled:
 

 
 
This is a 840 EVO 500gb SSD with NO OP, It will only allow operating system to be "over provisioned" 
 
 

 
Just imagine your boot speed if you can use it!!!!! 
 

howdy2u2
  GTXJB, (Sorry OP) Not to derail thread but..............Have you tried the Samsung Magician software on your XP941?

 
AFAIK the XP941 is an OEM product and doesn't have any software support for it.

howdy2u2

gtxjackbauer

howdy2u2
  GTXJB, (Sorry OP) Not to derail thread but..............Have you tried the Samsung Magician software on your XP941?

 
AFAIK the XP941 is an OEM product and doesn't have any software support for it.

Well crap!!! I would have liked to see what the 941 could have done with the software............. Not that it's performance is slow by any means even without running OP.

Simple.  Just run the software I ran, CrystalDiskMark or any benchmarking software for that matter.  Would I have like if it worked with Magician Software from Samsung that my EVO works with? Sure, but its not a deal break for me.  Trust me, its blazing fast.  My XP941 runs as if I had 2-3 EVO's in Raid 0, depending on the workload you throw at it.  If you were to have a M.2 PCI-E 3.0 x4 interface, you could get the SM951 almost double the speeds.  It is still slightly faster than mine on PCI-E 2.0 x4 but I didn't want to deal with returning it after the trouble I went through on other matters for it.  (Read bottom post for more info.)
 

 
If you decide with the SAMSUNG XP or SM, make sure you get your warranty on the product straighten out.  I went through hell figuring out who would take care of it.  Samsung sure wasn't since its an OEM product intended to be sold to PC vendors like Dell and the likes for their PCs and laptops.  I got mine squared away with newegg and had to purchase the extended warranty on it.

Thank you all guys.

mdzcpa
The Plextor is not quite as fast as the XP941, but it does come with a m.2 BIOS that makes the Plextor totally plug n play and a great option for the OS drive.  Something the XP941 sometimes struggles with.  It also plays real nice with either Win 7 or Win 8. 

Can you please explain this? In what sense does the samsung struggles and with what?

sahafiec
so which drive would you guys suggest as a OS drive only?

XP941.

Xp941 512GB will blow your mind, lol.  At least it did for me. SSDS in raid it felt.  Minutes was windows install.  Seconds less than 10-15, depending if you're doing windows updates etc will have your stuff up and running no matter how many things you're running in windows.  There's a reason why it has a premium price.  I can't wait till I get the rig up and running.

XP941 in the house, i ll come back with some benches in a bit :)

I got the 256GB model and running win8.1 pro.

astramonti
Thank you all guys.

mdzcpa
The Plextor is not quite as fast as the XP941, but it does come with a m.2 BIOS that makes the Plextor totally plug n play and a great option for the OS drive.  Something the XP941 sometimes struggles with.  It also plays real nice with either Win 7 or Win 8. 

Can you please explain this? In what sense does the samsung struggles and with what?

Just Google "XP941 boot problems" and you will get a nice list of the issues.  With newer motherboards it has become less of an issue, but as you look at the links Google provides you will see lots of folks still having issues.  The Plextor comes with a firmware BIOS and boots exactly like any SSD across any OS.  Win 7 being "5 years old" or not I didn't want to be tied to an OS because of my M.2. drive choice.
 
XP941 also has some disconcerting heat issues.  Some folks have had to add active cooling in some instances with cases with limited air flow.
 
Here is a good article about both of these drives.  There is some food for thought in there.  The XP941 speed comes with some trade-offs.  But to each their own.
http://www.pugetsystems.com/labs/articles/Samsung-XP941-Plextor-PX-G256-M6e-M-2-Qualification-575/
 

I 've put a heatsink on it and the side 240mm fan should keep it cool enough I believe.

Hey guys, what key does the mobo use to show the quick boot up menu during its boot? 
 
Edit: Found it, it's F7.