Hi David,
Right first thing, about the company, i have heard so much good stuff about their service, and say for instance they broke or whatever and it wasn't your fault i hear they're really good about replacements. As for build quality only time will tell (as i have only had mine about 2-3 weeks), but if it's poor build quality and they break, they will replace them no probs, they seem like seriously friendly guys. But generally they do seem well made, however, on that forum there seem to have been the odd person saying something happened their earphones, but it's probably a minimal amount of people that have had problems with them.
Basically have a look at that forum i linked you as its site is full of people who know a lot more than me, essentially they're headphone geeks
Or google the name for reviews, where professional reviewers have actually reviewed them, say in comparison to other ones. All i can compare them to are my shitty ipod stock earphones, but then again anything would sound better than them - more of a fashion icon i reckon!
I have heard the
Etymotic ER-6i iIsolator (but £72 on amazon)
http://www.amazon.co.uk/Etymotic-ER-6i-iIsolator-In-the-ear-Earphones/dp/B0007WTHLO are amazing headphones, but no real bass, ie probably good for non dance music. The isolation is meant to be better than the xbs that i mentioned but in regards to bass, it's meant to be very low, but mids etc really good. My opinion on them would be if you don't listen to electronic music they'd be perfect, but i do listen to a lot of electronic music so...
Have a look at this page:
http://headphonedeals.org/?cat=23
This guy reviews them comparing them etc
I was tempted like you spending double what i spent, thinking at this price £30~ the build quality won't be much, but personally i'm delighted with mine, and do spend a lot on technology and was well happy hearing the sound quality from a totally unknown brand (they're South Korean i think). Just look at all the positive words from that forum link.
Obviously if you spend double the money, the chances are you'll have better headphones, maybe go on that forum and ask the experts i'm willing to spend £x these are my requirements what do you say.
I know you want to make the right decision, it took me about 2 weeks to make my mind up, as i kept seeing headphones i liked reading a decent review, then the next review was like not good for bass, etc so the lack of bass put me off but eventually found the crossroads and would definitely recommend them.
Also, ultimate ears is meant to be a really good brand for earphones, but pricey.