Takeup is slow?
Duh, of course it is, for the simple reason that being connected to IPv6 (as opposed to being connected to The Internet) doesn't give any benefit for the two groups of people who matter: the users, and the content providers.
ISPs are a small band who sit in the middle, and can moan all they like about IP address depletion (because it will probably start costing them more to get IP addresses), but the users don't give a hoot.
Right now, if you go the recommended IPv6 deployment approach - dual stack - then you are connected to two networks. One is The Internet, and the other one isn't. It's just like the days when people connected to IP and X25, or IP and CLNP. The one which wasn't IP didn't have the content and services that the users wanted, so it withered away.
Even if by some huge stretch, just imagine that 50% of the content on the Internet was also available on the IPv6 network. People would still need IPv4 connections to access the rest of the Internet. So they've gained absolutely nothing by deploying IPv6 (*).
The pain and cost in going dual-stack is huge. Not only does your OS need to change, but all your network-aware *applications* need to be modified too. Sure, things like Cisco routers nominally support IPv6 - but try turning on the full set of features you need (let's say MPLS and IPSEC), and see if it works. Pain and expense without business benefit = no deployment.
Of course, content *providers* will have to remain on IPv4 indefinitely anyway, to keep themselves visible to the IPv4 users, which in circular fashion means that users who stay on IPv4 are the ones who benefit most: they can still access the whole Internet, and they avoid the costs. Any content provider who went IPv6-only would be suicidal.
There's only one way in which you'll get IPv6 deployment, and that is to embrace NAT. Treat IPv6 as a sort of super-RFC1918 address space. Run only IPv6 on your corporate or campus LAN, and have NAT/PAT gateways which let you talk at least TCP and UDP to the IPv4 world.
Unfortunately, the IPv6 nazis are so anti-NAT that they have decreed this is a forbidden approach - it's dual stack or nothing. They shouldn't be surprised, then, if nothing is what they get.
(*) Remember that IPv6 doesn't solve *any* of the problems of IPv4 - such as multi-homing, mobility, or security. IPv6 may mandate IPSEC in the spec, but without a global keying infrastructure, it gives you nothing more than IPSEC on IPv4.