It's pretty open. Maybe not as much as as GTA, but I'd hardly call it linear. Whenever you aren't in a mission you can just drive around and goof off. The driving is actually quite good. Reminiscent of NFS, very arcade-y. For the first time in an open world game, I don't mind the racing missions! There's a good variety of stuff to do outside the main missions, but the story is decent enough too.
It feels almost like an RPG-lite with the upgrade system and collectables/hidden objects that actually mete out rewards. Again, for the first time, I don't mind tracking this shit down. In fact, it benefits you to do so.
I don't know what engine the game runs on. Reminds me of RAGE, though the physics have been heavily tweaked to be more arcade-y.
The combat is pretty good too. Evocative of Arkham Asylum/City combat. And it's ridiculously fun just running up and down the sidewalk and flying jump kicking passersby.
Oh and the game keeps a running tally of your "records" (longest clean drive, most time eluding police, flying jump kick count). It's genuinely fun trying to beat your last longest clean drive record, and the arcade driving physics make it a lot easier than it was to do in GTA IV.