a must!
i went into st. paul's thinking it'll just be like another european cathedral but it was nothing but ordinary. it's by far the best one i've ever visited. the ceiling paintings are beautiful, and, a lot to take, in fact i had to sit down to gaze up properly so i wouldn't get dizzy!
the cathedral is of course huge but at the time we where there (about noon on a saturday) it was fairly empty of tourists which made the experience a lot better.
if you are physicaly capable i strongly reccomend taking the stairs up to the dome, the way up isn't that tireing as its devided to three levels, the first "floor" ends inside the cathedral near the ceiling and its a great place to stop and look down on it.
the second floor ends outside and has a fairly good veiw of london, this is also where i noticed most visitors stopped climbing up. the climb to the top from here is the most tiering but well worth it since the veiw on a clear day is amazing, also its not too crowded in that small balcony.