I went for a little over a week with my fiancee last December. I really enjoyed it. There is a ton of stuff to do and see in London (any guidebook can tell you most of these things... St. Paul's, London Eye, theater shows, pubs, shopping, museums, etc, etc.).
We took a couple of side trips... one short one to Windsor to see Windsor Castle, which was pretty impressive, and then a longer one to York. York was excellent, I definitely recommend it. The city is surrounded by a medieval wall,
York Minster is astounding, and there is a neat little pedestrian-only area downtown.
One thing to keep in mind is that London is a very expensive city to begin with, and coupled with the extremely weak dollar it is a VERY costly place to visit right now (although the same could be said for many cities in Europe).
Also, look into getting a "Visitor Travelcard". You have to buy these in the States before you leave (you can get them at some travel agencies). They are discounted cards that allow unlimited use of the Underground, most buses, and other public transportation in the city. We were very happy we bought them because our hotel was right next to a large Underground station so we never had to take a cab or anything.
Overall, I had a great vacation there. :beer: