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.
The same holds true for Silex[1], the Symfony Components-based micro framework. Silex and Lumen look very similar (not only because Laraval/Lumen use Sf Components as well), not only because both use (just as Laravel) Sf Components. Slim 3 seems to move in the same direction of modern PHP frameworks, where you return a response from your controller, instead of setting up some global state (the main thing I don't like about Slim 1 & 2).
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 ;)
I've done some projects with Silex and it's not bad but I've found myself losing quit a bit of time because you can never really use most components exactly like in the Symfony standard distribution but the docs mostly show you that way.
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've used "Silex" and "Slim" in various small projects and API's. Then I found PhalconPHP.com I prefer using Micro+Volt now. New version 2.0 will be released on April 17th, 2015 So check it out, you may like "volt" template engine more :-)
This is definitely a very cool feature and makes this a very good candidate for MVP's or prototypes. I'm not a huge Laravel fan, but this feature alone might sway my decision.
This is a serious advantage to other frameworks if this principal holds true. I think anyone working in PHP has had their tails burnt a little too often with all the framework churn.
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 ;)
[1] http://silex.sensiolabs.org/