The handling usually is improved by widening your track.
Warranty is like anything else, if there is a problem remove them before it goes to dealer. The only things spacers have been known to wear is wheel bearings.(I have never had an issue and run lots of spacers/adapters for fitment).
You can run them with your wheels and suspension no problem.
On the stock 18" M wheels I ran 12.5mm front and 17.5mm rear mainly because that's what I had from my 335xi Msport. You could do 15F 20R for an aggressive stance on stock suspension you should be fine.
People will run different sizes front to back due to numerous wheel options and offsets to get the required fitment. With my BBS wheels I run 17.5 front and 12.5 rear to get even fitment front to back and my fronts final offset is 14.5 and rear is 22.5.
Reliability is pretty good all around, I prefer the Turner Motorsports spacers because they have notches for easier removal and I have had good luck with them in numerous vehicles.
Hope this helps