That's 15 mm per axle stub, making 30mm in total. The wheels would clear the kit with 12mm spacers but 15mm come with machined lips to ensure that the wheel mounting is good.
In terms of the handling - the wider front track reduces the tendency for understeer and actually makes turn in a little sharper. It also has the effect of slightly softening the front suspension (longer levers) and reducing the centre of gravity.
All this makes the car a little more steer-able on the throttle, allows a little more power to be applied a little earlier in the corners and slightly higher entry speeds.
Wheel bearings seem fine after 14,000 miles in 17 months but I'm told they will wear quicker than stock.
Another benefit is that I no longer wear the outside edge of the tyre as fast as I did on a stock set up - wheel alignment was good both pre and post spacers.
So in nut shell, spacers are OK - a good compromise and a lot better than buying another set of wheels (I have 3 already!)