I've been looking for these! Aside from the fact that they're quite expensive, they don't sell 0.3 here in Sweden.
I love how fine and neat the lines these pens make, but the fine and very thin pin makes these quite fragile as well (especially 0.3). Just as long as you don't apply excessive pressure when writing, then you're fine.
I've been using 0.4 as well and they're just as good, although I still prefer 0.3.