Pros: SQ, Accessories, Fit, Cable, Isolation
Just like the title says: What Makes Them So Good? They Don't Do Anything Wrong
The W4s aren't going to blow you away with their treble shimmer or uber-deep bass nor their silky smooth mids. Their just-warm-of-neutral signature isn't going to make you break into song and dance at every available opportunity.
What they will do, however, is everything, pretty dang well. It's like shotgunshane says "The W4’s are supremely balanced and natural. No one frequency intrudes on another. Detail retrieval is unparalleled. They are very smooth and coherent. The funny thing is you may not like them when you first put them in because there is no WOW factor. Once you get over that initial disappointment, they seemingly get better and better. The best compliment is to say they do nothing wrong."
The clarity and balance is the best I've heard (granted I've never dropped over $300 for IEMs). They are equally adept at handling the glockenspiel and flute in Holst as they are at navigating the iconic sub-bass from James Blake.
The accessories are satisfying and the case works very well. It bears mentioning that the cables are the best stock cables I've seen. The braid is great and their strain reliefs don't make me scared to run with them.
If you're looking for multi-armature IEMs that hit the middle ground between super-neutral and ultra-fun, I can't recommend the W4s enough.