I have ruled out the X5's just because of how big they are.  I now am trying to decide between the Custom 3s and Image X10s.  A few things that are important to me:

-Comfort - I'll be wearing them sometimes for hours at a time.

-Durability - I'll be carrying my headphones in my pocket sometimes without them being in the carrying case and need headphones where the cable won't start to deteriorate after less than a year of use.  Don't get me wrong - I will treat them with extreme care, I just want them to be able to withstand being without the carrying case when I carry them with me.

-Sound quality is pretty important but honestly, I don't think I would be able to pick apart the sounds of the two headphones, considering my previous pairs of headphones have been $50 or less.

Based on these 3 things, which would be the best for me?

I think the custom 3's are (or were) considered their top of the line.  I own a pair AND I bought a second pair in case I ever lost/broke the first.  The spare pair is still sitting in my office drawer.

comfort:  For me, they are comfortable.  I've never worn ANY other so I can't compare agasint anything else.  There are times the inners of my ears do feel a little 'sore' (strong word to use) because of the pressure.  I attribute that mainly to me having never in my life used something like this and simply having nice, soft tender ears :)  (kinda wants to make you cry, doesn't it?)

Durability:  I can only speak to me...I'm sure someone else might have had theirs break in 2 minutes of receiving them.  I've had mine now for probably 2 years.  Use them over weekends and can use them for hours on end.  I use them with some hearing protection muffs on OVER them to kill the engine noise of my tractor or backhoe or chainsaw.  I took them on a 3 week trip to England, Italy and Turkey.  Had them on in the rain.  I keep my Ipod (it's the big one but I don't remember if that's 80 or 120G)...anyways, I put my Ipod in the C-3 case and place them in there with it, some might say I'm mashing them.

Sound quality:  pfffft....  don't even ask

Now...I WILL say that my cord has some ....what I'll call kinks in it.  One is right at the plug.  These DO annoy me as it makes me feel like the wires are very brittle.  I'm not bashing them, just giving my honest feeling on how it makes me view them.  That said, I've not had ANY issues with the wires, just that they 'feel' fragile and in fact, have a couple kinks in them.

I try to be careful with mine but I know there are many out there who are retentive careful.  I'm not retentive careful (careful winding of cord, placing earbuds carefully into open space inside case)....nope...  I tend to curl it up and mush them in there.  I do try to be gentle about it.

So, you can count me in as a VERY happy camper with my first experience with these things, I always look forward to using them.

Oh...  I think I use the large ovals.  Started out with the mediums and found at times, one of them would simply pop out.  I probably need something in between the two sizes.  Don't like using the double flange when I'm out getting sweaty as ...how to say this...  the angle of the earpiece (remember, I almost always have OVER ear protection on as well) seems to rotate.  When the angle of the earpiece rotates and I'm using the DOUBLE flange, it seems the output hole gets moved to the wall of my ear and it kills the sound.  The earpiece never really straightens itself out.

By using the 'round' piece, I can still get that change of angle cutout of sound BUT it will rotate back and I don't have to fiddle with it as much.

I don't know if that makes sense but it's what happens to me.

My wife and I each own one set of C3's.  Like Coytee we too have kinked cords....very kinked cords, it irritates me quite a bit so I'll stop talking about that issue at this point. 

For me the stock ear gells never sealed completely so I bought some Comply Foam tips for them.  With this combo I can unabashedly state the C3's are nothing short of incredible.  I'd like to try the X10's or X5's at some point.  I wear my C3's to work out, sit around the house, on planes....virtually anywhere I wish to privately listen to music.  Their confort is unsrupassed.  Their sound is amazing.  

I try to treat them carefully becasue of the cord issues.

Overall with the Comply Tips I am very soundly in the camp of the satisfied.

If Klipsch offers another sale I'l likely buy another set of C3's and a set of X5/10's.

 What is the cord of the Custom 3 made of?

If I had to hazzard a guess, I'd say teflon insulation underneath a cloth sheath.