Ok, so up until now, whenever anybody wanted advice on which flash to buy, I always told them (based on my previous experience) to buy a first party flash (Canon, Nikon, Sony etc) as it would meter better and colour temperature is known etc.
Now however I am looking for a flash. Im not a massive fan of flash, and so far havent found a need for anything other than the pop-up flash for digital. Now however, with the convenience of my 18-200mm, I find I take this lens out so often which poses some problems for flash. Most notably, its aperture is small and even with VR is tough to shoot in dim conditions. Also, zoomed out or focussed close, there is often a shadow of the lens in the frame. Also, the low power of the pop-up flash and bounce flash are more reasons why I want to get a proper digital compatible flash gun.
A friend over the last few days has let me borrow and test out his Nissin Di622 which he swears by. I was surprised to see that the flash experienced none of the problems I thought it would and turned out to be highly competent. Its actually cheaper than Nikon's cheapest flash, and is much more powerful, recycles faster, can rotate horizontally as well as vertically, easier to use as a slave flash, and reaches further above the lens.
As of this, Im actually going to buy this flash rather than one of Nikon's own. Its only for light use, nearly always going to be on camera so I have no need for anything higher upmarket.
Nikon D5000 and D300 with 12-24mm, 35mm f1.8, 50mm f1.4, 60mm f2.8 Macro, 18-200mm, 17-55mm f2.8 (all Nikon)