Like most things you put on your ears, you get what you pay for before the diminishing returns kicks in...
I like the Shure 325's for a reasonably priced dual-driver design, from a reputable company with a great warranty. The 5's are worth it for that
little bit more, but you've hit the point of diminishing returns.
A lot of folks on
http://www.Head-Fi.org like the Westones, but I've never messed with them. The guys on that site tend to know their stuff, though everything they say should be taken with a bit of salt (ok, maybe the whole salt mine).
Personally, if isolation isn't a HUGE issue, I'd still go with regular
open-backed headphones. My personal faves for working off a MBP are the Grado SR80e's - $100 and you have to pay at least 3 times that to get anything really "better". I've never found a cheap
closed-back phone I like better than the Sony MDR-6/whatever-the-consumer-name-is, around $150. You'll DEFINITELY get better bang-for-buck out of headphones than in-ears - they are all pretty inarticulate (IMHO and Experience) before you get to the $300 mark. The physics just work against them - you
really need 2 drivers to make a good, articulate in-ear, and that pushes the prices up...
The reason I recommend the Grados over Senn/Beyer/etc. is both price and that you want to work off the headphone out of the MBP. None of the other
good phones I have used work nearly as well off the built-in output than the Grados - just my experience, but they really are in another class when powered off a questionable amp (which
any built-in output is).
Anyway, good look with your search - I've been looking for a good "portable production" rig for a long time, and am finally happy with a Duet2, MBP, and the Grados. My home rig is the same, for the moment, but replace the Grados (which I still use as a reference) with a Lake People headphone amp and Sennheiser HD800's - before you say "yikes, $$$", I must say that I am coming from a treated room and Barefoots, and the HD800/LP/Isone/Apogee rig is
almost as good for 1/8th the price!
Oh, and speaking of Isone, I highly recommend it for getting a "more realistic look" at working on phones -
http://www.toneboosters.com/ - it is a "speaker and room sim" that is cheap and effective. No affiliation, just a fan of the product - and it is cheap!