I've been using Sony MDR-EX70 earphones with my MP3 player for several years now, and for the £30 they cost, I thought they were excellent. I kept toying with getting some Shures or Etypmotics to replace them to see what benefit the extra money gave, but never got round to it. After Xmas, I had a bit of spare cash, so I bought a set of E3s.
When they arrived, I was distinctly underwhelmed - no bass whatsoever compared to the Sonys, and a very forward treble. I tried all the rubber earphones supplied with them, and none made any difference - I was beginning to feel I'd wasted a lot of money.
I then tried the foam ear sleeves, and there was an immediate improvement. The bass wasn't as loud as on the Sonys, but it was at least there. I ordered a large bag of foam sleeves, as they look like they will need regular replacement, and decided to persevere for a bit.
One very important point - these earphones sound dreadful out of the packet, but they improve enormously with a week or so of use. They now have clear, well-defined bass and a nicely integrated treble which is detailed without being shrill. They sound noticeably better than the Sonys, and I don't regret the purchase at all. Be warned - they are brutally revealing of poor source material - all the drive noise, hum and crackles from your MP3 player will be clearly audible!
The only downsides are that the foam sleeves (which are a must to get the best sound quality) are disposable and may not last too long (and cost £2 a pair for replacements), and the stiff cable makes a lot of noise when it rubs against clothing etc when you are walking. Other than that, they are a very worthwhile upgrade to the £30 Sonys - the extra cash makes an audible difference.