When I went down there I contacted Blisstick directly and they hooked me up with a new boat for pro-deal equivalent prices. Then one of the workers drove it across the north island for me to pick it up! Super friendly. They also have a program where if you work in the factory for 50 hours, they give you a better deal, and you get to make your own boat. At least these are the deals they had when I was there 3 years ago. Awesome people.
I forget the name of the book, as it's in the storage unit, but the guy who runs the kayak shop in Murchison (mentioned in the post above) was also involved in writing a fantastic guide book to the whole country. Kayaking New Zealand or something- rather generic title that escapes me, that is a 'best of' 100 runs for both islands. I recall his name was Mitch? Older fellow who is an amazing paddler and really cool guy.
Buy the book (if you can't find it based on my lame description PM me and I'll go look for it, plus I can dig out my old notes if you like) and it really has all the info one really needs.
While there are runs everywhere, if you're strictly there for paddling, the south island is definitely the place to go. Hokatika has tons of stuff, though a fair amount of it is stout. I agree that Murchison is a great place for class 3-4 stuff.
Lots less people in the south. More of the common touristy stuff is on the north island, though quite a bit less paddling. Plus, most of the north island stuff is rain and seasonally dependent (if I recall correctly anyway). The south island stuff is rated by: do it before the rain, do it while it's raining, or do it after it has already rained as lots of it is glacier fed and can come up (too) fast. Keep in mind, the south island west coast sees over 21 feet of rain a year (yes, feet).