The main problem I have with being a specator is how hard it is to move around London, especially when you want to get from your viewing point to the finish to meet your runner/s.
Access to some stations is difficult because you can't cross the marathon course (or not easily, anyway), and underground stations near the course are often closed, as they would get too crowded for safety.
Even though you can get the DLR to Tower Hill, getting on the underground there to get to the finish can mean waiting in a queue to be allowed to enter the station.
Personally, I hate it - but do it to see my friends run. I would not take small children.