Having also owned this product's predecessor, the NE-6, I was able to make a direct comparison between the two. Since most people read these reviews to decide whether or not to buy their first Nuforce headphone or upgrade from a previous Nuforce, I'll answer both questions.
Question 1: "Should I buy these? I've never owned Nuforces."
I own headphones spanning in price from under $50 to around $1000. There's a law of diminishing returns with headphone sound quality, however--while a $60 headphone is likely to sound much better than a $30 one, a $600 headphone is not necessarily as drastic an improvement over a $300 one. For the headphone buyer on a budget who is looking for the best possible sound for under $100, you've found your headphone. These have great, tight bass (almost a little too much at times) and offer very refined mids and highs for their price range. They are also well-built and likely to serve you well for years. I've owned a lot of sub-$100 headphones, and these are about the best.
Question 2: "I have the NE-6/NE-7M, etc. Are these worth the upgrade price?"
The greatest improvement I noticed going from the NE-6 to the 700X was in the mids. I feel that the improved drivers offer better clarity, tightness and realism in vocals and midrange instruments. The bass quantity and quality seemed about the same to my ears (pretty substantial and tight for this price range, but borderline flabby on certain songs), and the highs were perhaps a bit cleaner and less tinny. I consider the price increase over the 700X's predecessors to be justified when the improved sound and build quality are taken into account.
In conclusion: whether you're upgrading or new to the Nuforce family, these are a great buy.