It seems that the new trend in "co-working" in NYC at least is towards startup focused spaces (e.g. Dogpatch Labs, General Assembly). Granted, the turn over may be even lower than the average co-working space (no numbers, but at Dogpatch NYC it's 6 months+ before you need to start looking for other space - there is actually going to be a fairly large turnover in the next month or so) but there is something to be said when everyone in the space is working on a company rather than just projects. It'll be interesting to see how these spaces develop, but from my experience at Dogpatch NYC, personally, I find them to be a great place for an early stage company.