Visited St. Paul's Cathedral last September 2012 (so a little late with this review!) with my husband as part of our honeymoon tour of London.
The cathedral is a stunning building and very much worth the entrance fee (although we used our London Passes to get in) just to marvel at the beauty of the place alone. The outside is magnificent but the interior is awe inspiring. It really is worth exploring all that it has to offer though as you won't be let down. Most interesting was the dome and the beautiful artwork and statues scattered round the building.
The real highlight however was the crypt where you can view the tombs of Nelson and Wellington (as well as many others including the architect of the building, Sir Christopher Wren). My husband especially enjoyed the crypt as he had always wanted to see Nelson's tomb as he's a huge fan of naval history. It was good to finally be able to take him (I had seen the tomb before on a previous visit).
We spent a few hours here and really enjoyed our visit. It's so much more relaxed and friendly than other places like this in London (Westminster Abbey!). Would visit again.