Well the first bad point, if you can call it that, is cost! But if you've looked for any prices you will know all about this!
As for the ride quality, well the higher-spec the coilovers then the more comfortable it can be. But in short, you will feel most of the imperfections in the road and it will feel bumpier than on regular suspension. There is less travel in the firmer springs of coilovers so it the bumps are transmitted through the suspension far easier than with regular OEM suspension. Purchase fully adjustable (ride and height) coilovers and you will get more ride comfort than on fixed-damping kits.
On your current 15" wheels and what I presume to be 195/45 tyres, you can lower up to 60mm from standard height without any scrub or rub. And still negotiate the majority of speed bumps!
No idea if it will be harder to turn, I haven't driven a lowered car without power steering yet...
Cheers