I have added rangefinders to my arsenal only recently. The truth is, this is the first type of camera, I am comfortable taking out with me each time I get out of my house, every day.
It will be lacking in the obvious situations: when you want to control your dof, esp with close subjects, when you will want a tele, when you need a ND grad folter or similar.
However, as 90% of my photography is in the 25/90 mm range, I find myself very happy using a RF, over an SLR. When I go out shooting "light", but seriously, I would bring a couple of RF's with a wide angle and a 50, plus a SLR with an 85.
Honestly, using lenses over 50mm on rf's is not so attractive.
ZI is a wonderful camera, and ZI lenses are really great. I use Zeiss lenses on Hasselblad and the ZF line on Nikon, and the ZM line probably beats them all.
If I were you, I would definitely keep an SLR with a macro, a longer pertrait lens and a tele, and would get a ZI with the 35mm Biogon to start with. You will discover a new degree of freedom, new way of framing, and new speed of shooting.
Take a look at my flickr page (the link at the bottom) there are 3 groups of shots from Warsaw , the first two mainly with rangefinders, and the last, from the cemetery, mainly with the Nikon and 50/2 Makro Planar - you will get the sense of what I have just said.