The drop in functionality right into the full framework is fantastic. Really like the ability to write API's with much less overhead, which was a factor that came into play for me recently. Nicely done Mr. Otwell.
For now, Silex is still my favorite, as it has the temporary bonus of having more providers available to integrate 3rd party libraries. Sure, I can drop-in whatever library I like, but having someone do the setup (configuring Twig alone can be a burden if you want to get it right) just makes it easier for me ;)
Also you have to be careful with dependencies version or you're quickly in Composer hell (because Symfony moves forward so fast).
Always wanted to compare it with Slim but didn't take the time.
I'm glad its not just me. The documentation gave me problems. I started wondering why I didn't just use symfony in the first place.
It also helps me keeping current with the framework since it's currently all the rage in the (French) PHP job market.