Pros: Customizable fit, good seal, great audio range
Cons: expensive, cord not replaceable
I first heard of Westone when standing in line for the iPhone 4. A guy in line had a pair of Westone 3's and let a few people try them. I was blown away, could not believe they were that much better than the BOSE IE I was currently using. After discovering the price, I put any thoughts out of my head of purchasing.
Fast forward a few months and after a stroke of good fortune, (the cat eating though the cord of my BOSE IE's) my wife said I should replace them with the Westone's (did I mention I have a great wife :) )
They come with about a dozen different ear pieces you can fit on the monitors, allowing most people with normally shaped eardrums to find the perfect fit and seal. A nice molded zippered case is also included for carrying.
As far as SQ goes, I would classify them as general purpose. If your listening covers many different genre, then they are well worth trying out. If you're a hip/hop electronic only listener, there are probably better options.
With the introduction of the Westone 4, they're no longer the latest, greatest but they are money well spent... if you can justify $300 plus for IEM's.