I've been the biggest SE fan for years--every phone of mine has been an SE.. and naturally, I wanted the X10 badly.. especially when I can use it for 3G on T-Mobile.
I tried really hard to keep convincing myself that it would be worth it, but I was honest with myself and couldn't justify buying it in front of the Nexus One.
Nexus One will continually get the latest and greatest of Android, and has been rooted for maximum 'tweakability.' Let's face it, N1 is future proof, and X10 is at the mercy of Sony Ericsson's legendary painfully slow software support.
If SE can get it right with an X10 successor, that might be worth switching to. For now, it's Nexus One.