I own a December 2013 model and drove it with the original setup for approx 3 weeks after that I lowered the i3 using H&R springs.
The wheelbase of an i3 already is pretty short which makes it not the most thinkable confortable car. If you are putting stiffer and lowered spring on it it is simple > you will loose some of the little comfort the i3 owns..
Lowering the i3 gives more or less the same result as to any car being lowered, you loose some comfort and get some more 'sporty' driving experience back as well the better looks.
The i3 lowered yes is a bit more bumpy but here in the NL not really noticable compared with the standard setup, in general we do have nice roads over here.
The big negative difference, big negative you will experience is speedbumps these you really need to take with lower speeds than with the standard setup.
Advantage of lowering the i3 from my experience;
-Less sentative for (side) wind
-More sporty ride, cornering goes better, faster (but thats my feeling, nothing measured)