If I was doing all this again, the first question I would try and answer is; "How many flash guns will I ever want use at one time"? If the answer to that question was, "One and only one", then I would just go straight out and spend the 60-80 quid required to get a reasonable third party, iTTL flash - job done

!
As hinted at already, what you're getting with guns like the SB-900 is a window onto the wonderful world of Nikon's Creative Lighting System and if you ever plan to use off-camera flash with IR triggering, or even two or three flash guns at the same time, then this is a really convenient (if expensive) way to do it

. You can still have those functions with a non-Nikon flash, but usually you have to dial in the settings manually, as it won't meter everything for you.
There are other benefits with the SB-900, over the cheaper alternatives (flash zoomable to 200mm, balanced fill flash mode, diffuser, gels and carrying case included in the price, etc.), but a lot of owners probably never really experiment with those things anyway :|.
IMHO, spending 350GBP on an SB-900, only to leave it stuck on top of your DSLR with no brothers or sisters for company

... is somewhat extravagant.
For a single flash set up,
or a five or more flash gun set up (where you're talking about 2k to stick with Nikons

); I'd be tempted to go and look at
FlashInThePan's Equipment for sale thread and see what guns he has at the moment

. Just my $0,02