Very nice review! 
I bought my wife the FiiO X1 and a pair of Shure SE215K's for Christmas and she LOVES this player and now uses it every day. She told me that she was happy using her smartphone as a portable music player and would never use a separate DAP. Its always a good sign when your wife/girlfriend hears the difference in sound quality and is impressed. I tried it out for a few hours with my Westone W40's and B&W P7's and I'm of the opinion that it sounds better than it has any right to for the price. Use the Lineout to the FiiO E12A and that bumps it up several notches. FiiO hit it out of the park with the X1 at its price point. 

A good to the point review. 
 
On the con, what line out? 

Looks like a Sansa Fuze on Steroids, with a proper line out. That's a good thing, in my opinion, and in nice aluminum. Thanks for the concise and helpful review.

The headphone out can be switched to Lineout via the menu. 

Thanks guys! I'm glad you all find the review helpful.

Nice review! and by any chance i just bought this DAP too XD
its line out sound really amazing with it's balance between vocal and bass i tried to connect it to the AUX port of my Aego M Multimedia Speaker and from my opinion its just lack at soundstage width(not as wide as i hope) and high frequency sound terrible(like hard rock or heavy metal song)

Honest and Great Review.  I too, now am the proud owner of an X1, and am glad to have something a bit more user-friendly as adding music is concerned.  I am still learning the ropes, but paired with my M-100's it sounds great.

If anyone is planning to use the X1 as a standalone player, finding the right synergy is key! For example, I found the X1 pairs really well with the T-PEOS Altone 200. I don't even feel the need to use a separate amp.

Very good review. I am using x1 with GR07 VSONIC. Its awesome. Soon I shall be getting topping NX1 amp.

Nice review, and (good for me) I have had the same impression after first 20-30 hours of listening of my X1.

Good review. I also have a new FiiO X1 and like it.  I have a cable question.  This is my first hi res player.  Does using a high quality aftermarket cable for transfer of files from the computer to the player make the sound better?  If so what do you recommend?  PM if you like.

Any USB cable that works with the X1 and your computer will do just fine. No worries. 

Excellent review. I am going to check it out. Thank you

It doesn't play WAV files well. It skips songs frequently. And, there are POP and HISS sounds between songs, at times.

How is this better than say, a Rockboxed Sansa Clip+ (or a smart phone like a Note 2)? Much cheaper and the audio quality on it is bar non better than an iPod (at least from the 3.5mm jack), has no problem driving a set of M50's. The battery life also seems quite anemic at 11 hours. 
 
Also, does it support playlists in .m3u or .m3u8 format? The design/build quality of it does look quite nice at least.

How amazing this little player is price/performance wise was made clear to me the other day. I liked the X1 so much I wanted more of the same, so I ordered an X5. The X5 was a disappointment. Sound quality...well you could easily fool me into thinking they're the same internally. I keep A/B ing the two with HD650's, K701's, RE0's and SE530's, and I can MAYBE spot a hint more detail with the X5, but that could be the placebo effect.
 
The X5 does play louder than the X1, the X1 doesn't have enough oomph for HD650's and K701's, but plenty for IEM's.
 
So why get an X5? The X5 is bigger, heavier, has the same battery life, the UI is slow, everything you do has a two second lag, the EQ doesn't work with high bitrate files and barely works at all...
The answer is...don't. Unless you insist on using it with insensitive headphones, or you absolutely need 2x128GB microSD cards, OR you need the DAC function...there's no point in paying 3.5 times as much for the X5.
 
Just goes to show how amazing a deal the X1 is.

@spom The Sansa Clip+ is a notable player and is a great beginner's player for anyone starting in this hobby. I still feel that it needs an amp tho... technically it's capable of driving most IEMs and sensitive headphones but you can really tell how it scales well when paired with an amp. I have the Phillips Fidelio L2 and it sounds marginally better with the X1 than the Clip+. I'm guessing the amp section of the X1 is just more capable than the Clip+. That's one of the reasons why I didn't compare the two in this review. 

Nice review will try to pair the X1 with a good amp one day. Pairing bright sounding earphones/headphones with the warmer sounding FiiO X1 really balances out the sound performance nicely like yin & yang though =P

Very nice review! You might make a comparison in sound between the e11k and C5D from the line out in x1 ?

@JuanLuis91 thanks! I purposely didn't make a comparison because it'll be more of a difference in sound signature between the two amps with the X1 becoming a non-factor. Both work seamlessly with the X1 line out and for one to prefer the other will more likely depend on preference. Only difference I can substantiate is the different approach in bass boost. C5D bass boost works better for IEMs with sub bass roll off while the e11k bass boost is perfect for IEMs with flat and well extended bass response as it only adds a bit of thump/slam and warmth. The C5D bass boost can sometimes be too much for IEMs that already has bass emphasis.