I may as well tell you the story behind these earphones and me. I purchased a 16GB Sony S-Series Walkman to replace a smashed up iPod. The Walkman came with a pair of Sony EX85 earphones, which are pretty decent. Well actually, I had never had earphones so decent.
When those earphones broke and I had to go back to a pair of 10 Euro Panasonics, I noticed a huge difference in my audio pleasure. So I decided, while waiting to see if Sony would fix or replace the EX85, that I would splash out on these EX300.
And I haven't regretted it one bit. Pros, they have a really sharp, clear and quite full sound. You would definitely notice the difference over a cheaper pair. Yes, I noticed I was hearing sounds in songs that I hadn't heard with cheaper pairs, as well as other little things; for example in one song what I had always thought was a drum thud, now turned out to be definitely hands clapping. After wondering for ages whether it really is worth investing in proper headphone upgrades, I can now confirm it definitely is.
Cons, well the only issue I have with them is that the cable is split into two parts; meaning I find it is either too short without the extender, or too long with the extender. But that's not such an issue that I'm going to deduct a star.
Like "Rich", I'm also now wondering what the EX500 sound like, or even the EX700. I don't know if a pair of Sennheisers in the same price range might be a better bet, all I can do is relay my experience: out of Sennheisers and Sonys that I have bought at a lower price range than this, I always preferred the sound on the Sonys.
If you've just bought or own an MP3 player and you want to upgrade your earphones to something better, then I think these are definitely a good option.