Here's my thoughts on the KWs..
Pros
1) Best coilovers you can get out there for street applications for our cars
2) High quality, lifetime warranty
3) Very comfy esp over smaller bumps
4) Less roll on directional changes, more positive steering feel and control
5) Recommended way to lower the car, vs. just Eibachs as the stock shockers will go fast with just the lowered springs.
Cons
1) Takes a few rounds to the shop to "tune" the setup from dampening to right height. I've gone in total like 5 times to adjust them over time. Good thing is you can get them to your exact taste.
2) Slightly more noisy, esp air release sounds when going over humps etc.
For me its only 2 options. Either the stock BMW M sport/Performance suspension or KWs. You might want to explore the new BMW Performance suspension as its designed by BMW performance and so far good reviews on E90 post. I think Velocity has it on his 335 and is very pleased with iti. Less headache, full warranty but price wise, its similar or more than the KW v2s.