The company I was with (yvesilena, the_alchemist,fluffymark and a couple of others) was lovely. So was the weather. The atmosphere was...fine. I think it was probably worth going. Some photos of yvesilena and the_alchemist on one of the lions in Trafalgar Square are here
But I couldn't help comparing it with the February 15 protest, and finding it wanting. I'd prepared myself for that, but that didn't stop it from rankling.
Edit: For a more positive perspective, see yvesilena's entry