At 99 Euros, I'll let someone else play with this first.
It looks pretty interesting, actually. I'm not sure I would go with one of these over something like the Lantronix XPort (at around $50).
There are many, many ways to connect small devices to ethernet these days. I'm not sure there is much of a market, yet, however. It's one of those "it costs too much, I'll wait until the price comes down" vs. "We only have a few customers, so the price has to be this high" situations. Once the market takes off, I think we'll see a potential explosion of devices available (unless ethernet has been replaced by then).
Would you pay the extra $100 for a refrigerator that you can log into to check its temperature?
In many ways, I think these benefit the tinkerer/hobbyist the most. They allow you to create ethernet-enabled devices relatively cheaply (if you're only making one or two). At that level, you don't always need to justify the cost.