Have a couple of good-sized ones and you will see the difference. Ours are good on the landing but the one in a dark bedroom is a bit disappointing - you need more light in a working room I think, i.e. one of the bigger sizes. Definitely get the sort with sections of straight mirrored tube rather than a flexible duct.
The Sun Pipe/Sun Tunnel product works really well if you make sure you use one of the top products, do not use flexible ducting systems as this really dosn't work. You can mount them on either north or south facing, obviously south facing aspect is much more effective. Check out www.solarspot.co.uk. I purchased one from these guys and they were incredibly helpful and took care of supply and install.
We renovated a house 5/6 years ago and put a sun pipe on the landing. It really did a good job of bringing natural light to the space. Would not be brave enough to do it myself though, but if you have the skills go for it!
We have one in a shower room and it is pretty good. Ours is not ideal as the pipe had to bend passed a chimney and then is in slight shade from the chimney, but was still worth it. Not sure about installing on a north facing roof, might not get enough sunshine.
I'm thinking on getting one on my landing (no natural light). Velux one is £250 for kit approx I'll get a roofer to do it though as I don't fancy my roofing skills & they've got all the tools etc. My local Loft Co shop sells them.
Living in a modern bungalow at mo, the kitchen is north facing and seems dark and horrible. Thought of a sun pipe anyone got one? Will it work well - just one? Cost? I'll saw though and sure you can do most yourself. Imagine the pipe comes with the kit for fitting through tiles.