The AH-C300 comes with a one-year warranty; proof of purchase or sales receipt from an authorized Denon dealer is required for warranty claims.
As always, price is worth considering. While $250 is well within the price range for well-built premium in-ear headphones like the Denon AH-C300 (you can find them online for closer to $210), it's easy to find similarly equipped headphones for $150 or -- though they certainly may not be as solidly built.
It's hard to not notice the AH-C300's exceptional bass power, and the bass emphasis is balanced with a fair degree of treble "sparkle." These headphones' sound is far from neutral or accurate, but that's exactly the point: the AH-C300 was designed for maximum enjoyment of bass-heavy music. In other words, the AH-C300 probably won't appeal to buyers with audiophile tastes.
The bass fullness extends over a broad range, and yet the AH-C300 doesn't sound muddy; bass articulation/definition is good. While I was listening to electronica with lots of bass, the AH-C300 had more and better low end than the $180 Bowers & Wilkins C5s or the $400in-ear headphones did. Those headphones had clearer overall sound, and the C5 in particular had wider, more "open" stereo imaging, but I preferred the AH-C300's bass, which had plenty of texture and detail.
The Denon also played louder at the same volume setting on my iPod Classic than the other two sets headphones, and the AH-C300's sound was more dynamically alive. The C5's midrange was better, so vocals had superior presence and clarity.
The AH-C300's sound got better the louder I played it; it never sounded strained or harsh at high volume. Isolation from external noise was average for universal fit in-ear headphones. The AH-C300's bass fullness was less apparent in noisy environments -- such as the NYC subway -- than it was in my apartment. I've noted that before with other bassy headphones; the bass doesn't seem nearly as exaggerated because it's competing with a lot of low-frequency thumps and bumps in the subway and on buses.
Simply put, the Denon AH-C300 is a bass-lover's headphone. If that's what you're after, this Denon is the one to buy, but anyone seeking a more balanced, more neutral sound that can bring out the best in all types of music should look elsewhere. I didn't find the AH-C300s all that comfortable; I was always aware of the bulky earpieces dangling out of my ears, but others might not be bothered by that.