Until quite recently I haven't heard of anyone having a problem with supporting v4 only. But that time is ending; the first problem I've heard of is a VoIP provider who has problems with customers on DS-Lite. In about three years it's quite possible that IPv4 has the sort of occasional tunnel/reliability issues that IPv6 had five years ago.
"Google works, so it's not my connection at fault, it must be your site that's broken."
The big reason to support IPv6 now is IMO that (all else being equal) it's better to add v6 support when you roll something out than to introduce it later.
At work (a larger enterprise in Europe) we already see quite a bit of pain with IPv4. B2B connections are increasingly not using globally unique addressing anymore, so we often need to use prefix NAT and application level proxies to bridge clashing address space. This in turn is a support nightmare and is hurting reliability.
Our network guys seems to love the extra complexity, though.
I hate VPN-ing into other people's networks, it is such a pain-point. As you say, you have address/space clashes, and all kinds of other (sometimes even security) problems.
It sounds easier than it is, the main issue is scaling. Need to connect to one other person for B2B? Trivial. Two? A little harder. Ten, twenty, fifty, ouch...
