Wish I was going but alas. I visited three editions of SES New York and each one was spectacularly useful. The first time I went I thought I was decent at SEO, but the conference made me realise how little I really knew. The second time I went I thought I was on top of things, but again I was shown how far I still had to go. And the third time I went (in 2008) I finally understood that in SEO you're never done learning, there's always more.
From what I heard about it, SES London had a bad start but is now a world-class event. I'd highly recommend it, you'll definitely get your money's worth out of it. I'll definitely be going to the 2011 edition myself.