Like skwid, I was somewhat staggered at the £14 entrance fee - however, unlike him, we paid the money and went in. We spent an amazing four hours there, going up past the Whispering Gallery, right to the very top of the dome - and what a view awaited us - a far better and more extensive view of London (and beyond) than we got from the London Eye.
One caution, you need to be fit enough to scale the 550+ steps, but the reward for the effort is more than worthwhile.
On top of all that is the magnificent artwork, as good as any cathedral in the world - and all the history too.
A great experience which will linger long in the memory.