Hey
Ok. I have been using pure D3D10 for making my demos for quite sometime(about 2.5 years now). I think it is time for me to get into known engines, instead of using my own framework, because, till now, i can't do any kind of indie projects with anyone because they all use existing engines, and no one uses pure D3D10 anymore....
I have done some research, and i think the top free, mostly used, engines out there are: UDK, Unity3D, C4Engine. Please correct me if i am wrong.
I was looking for an easy-to-use engine, with a lot of functionality ranging from smart AI's to a good variation of Post-Processing effects.
If there are any books to recommend for me to start with the corresponding engine, please refer me to them.
Let the engine wars begin 
Like you I was in the same boat, i did Dx9 for years "i refused to use DX10, as it was locked to vista, and from what i gathered, people TRIED to avoid vista" anyway.
i went from DX9 like you; to a few engines, C4Engine was trash to me. Leadwerks was a lot of tun, i even bought the licence, it was also the engine i found
easiest to make look the best but when my LIC was revoked for no reason and the author is a known B^&$# i abandoned that after 6 faithful months. i suggest the same
Ogre3D is a great engine, i didnt stay with it long but that was fun too.
I actually left ogre for unity because of a project ive joined. and i gotta tell ya, i wont look back. ALTHOUGH i did prefer leadwerks.
theres just something about that pure C++ i cant let go of, unity, while fantastic. is like the Windows of the engine world. at least it seems to me that way.
With that being said, windows is EXTREEMLY productive, like it or not haha.
for the record unity is the only engine "if it can technically be called that, its more like the the RPG Maker of old"; that allowed me to get a full game goin in a day.
