NZ has been mentioned a number of times on this thread. I live there.
Until 2003 brothel-keeping and contracts for the supply of sex were crimes, as were soliciting etc. So, the law was stricter than the UK but it was a dead letter; sex work and brothels were no secret. In 2003, these laws were repealed and other laws brought in to regulate the industry. Whether one classifies that as decriminalisation or legalisation is irrelevant: the distinction has no meaning in law here. It remains illegal to work or employ someone below the age of 18, brothels must be licenced, and safe sex is compulsory. Someone mentioned above whether a sex worker could be sued for breach of contract (ie, for not going through with it). The answer is of course yes, but in reality the only remedy would be a refund, just as with any contract for services. You cannot oblige a sex worker to specifically perform a contract any more than you can a painter or an accountant.
Sex workers' earning are taxable, however, they always were before: earnings don't have to be legal to be taxable, and I expect the law in the UK is the same.
The law reform was brought in while Helen Clark was PM: certainly the best PM this country has had in my life and a better one than any the UK has had in the same time. It was not a government bill, but Clark did make it known that she expected her MPs to support it (and they all did).
The reform is generally considered to have been a success, ie, it is thought to have made the profession safer and better regulated. There is no impetus to rethink this. The feminist lobby has little to say, and what it does say is that current arrangements are best for the sex workers and should therefore remain as they are. I can't recall anyone advocating a change to the Nordic model.
I believe that sex work and the adult entertainment industry is less stigmatised than in the UK. The attitude towards sex work is pragmatic rather than ideological. There is much more of an attitude that if people want to sell or buy sex, that's a fundamentally a matter for them, and that the people concerned are not obliged to uphold any wider moral standard. The feminist movement here tends to uphold individual choice more than elsewhere. In other words, the classic liberal view, one which seems to have become old-fashioned in all manner of contexts up in Europe. There are certainly very few people who would condemn a disabled man for hiring a sex worker. I think there is also a somewhat more relaxed attitude towards men who visit adult entertainment establishments. I know very few who have admitted to frequenting or working in them, but that's probably a reflection of the circles I move in.
There is less of a problem with trafficking, probably because of NZ's geographical isolation. That said, there is stigma and there is trafficking. The extent of this is hard to know. Because NZ is a small country, investigating societal trends is comparatively expensive and therefore quite piecemeal. A few years after legalisation Otago University produced a report that said trafficking wasn't a problem: however, as the report didn't survey non-English speaking sex workers, it's hard to know how that conclusion could safely be drawn, and there are recent news reports to the contrary. There are a lot of run-down "massage parlours" around and about, and I suspect the staff are from abroad and are working illegally, and probably in poor conditions.
Personally I think it's impossible to know whether legalisation has been success or not. In my view, the purpose of it was to relieve the police of the responsiblity of enforcing a dead letter, and also because the previous law didn't assist the protection of vulnerable women.