I just purchased these at £13.
At this price I couldn't believe they'd be particularly good, but I needed some closed cans for the office.
I didn't need to keep external sounds out - rather I wished to avoid annoying my colleagues with leaked noise, which both of my current cans suffer from terribly...
I regularly use two other sets of headphones - the Sennheiser PX100 (£20-30), and the Sony MDR-SA1000 (About £100). The former being excellent portable headphones, the latter being Sony's entry-level audiophile cans.
In my opinion, whilst (understandably) not as clear and defined as the SA1000 cans, the XD100 certainly manages to match the PX100 in clarity - even offering a slightly more defined mid-range and bass. I'm very impressed with the quality offered in such a cheap set of headphones.
As hoped, the XD100 are pretty good at preventing sound leakage - however they will not prevent external noise from being heard.
They are also surprisingly well made, considering the price. Very chunky yet light-weight, with thick cord and a strong headband. I have no doubt that these will last a long time.
My only minor disappointment is that they are larger than the picture led me to believe. They're actually not much smaller than the SA1000, making them only moderately portable. As such, I'd suggest the PX100 over these if you plan to use them with an MP3 player - these are simply too chunky to carry around, despite their lightness.
Otherwise, for use in situations where you'll have a backpack they can be stored in (or are simply at home), I'd strongly recommend these to anyone on a tight budget. They come with an adaptor for hifi and home cinema systems, which they'd also handle well.