We just bought a Sapphire Black/Sandstone Beige Leather 28i. After about 700 miles, it is fantastic. I have no reference experience as I never even considered the 35i. This is the wife's car and it will be driven 85% around town and by her. Coming out of a Yukon Denali, the 28i felt quick and we absolutely love the fuel economy already. Again, no comparison here other than the 2011 we considered as a CPO with the 6 cylinder, the 4 banger seems much smoother than I expected.
I just returned from a business trip and was able to rent a new Caddy ATS with the turbo 4. Needless to say it was nice getting back ino the BMW this morning and dropping the kid off at school. The ATS was 'nice' but the 4 was rough and loud. IT shows that BMW has been building and selling this engine for a while and it's not their first time with the 4s. I am happy with my choice and look forward to when I can trade my C300 on a F30 328i.