I have two pairs of good headphones, one for in the house, and one for out and about.
As good as in-ear headphone technology is these days, I don't think anything can compete with a more traditional over-ear design, a proper pair of cans, it's simple physics IMO. If you're sat in your house listening to music via in-ear headphones, you're doing it wrong.
For in the house I use Sennheiser HD595s, but they're not cheap (around £100) and they're totally unsuited to use outside the house, they also leak huge amounts of noise, so they're very anti-social even in the house. (I only use mine if the missus is in bed or she's doing something on her laptop with her headphones on, I can hear my 595s down in the kitchen (one floor down) when I've got them cranked.) They also take a while to break-in (50 hours plus) and you'll need a 3.5mm > 6.3mm adapter LEAD (not just a straight-through jack) or you
will fry whatever you've got them attached to.
The payback for all that faffing about is audio fidelity the likes of which you wouldn't have thought possible through headphones -
http://www.audioreview.com/cat/headphon ... 50crx.aspxAs for in-ear headphones, iPhones and the like are pretty shit as players go, and you'll generally be listening to compressed music files anyway, or just playing games. I wouldn't recommend spending much more than £50, but it's certainly worth replacing the appalling headphones that most devices come with as standard (iPhone/iTouch/iPod included). The Shure SE110s are a nice pair of in-ear headphone and shouldn't cost much more than the £50 mark, you can spend a lot more than that but you're on a curve of diminishing returns, especially if you're just listening to bog-standard MP3s.
For your £150 you could get the Sennheisers and the Shures!