I've just started doing core work in the last month. Nothing major, just sit ups. When I started I could only do 12 and got back strain.
So obviously I had quite a weakness! Now I can do 40 (which still is pretty average etc but anyway). It hasn't made me any noticeably faster on the bike so far (hard to tell though as my leg strength, heart and lung fitness is still very average, being very early in the season here in NZ), but has meant I can keep seated on steeper gradients without straining my back. And it has helped me remain comfortable post 3hr ride, without the usual stiffness, lower back muscle fatigue, and back spasms.
Core work my not make you any faster on the bike, but it definately seems to help post ride which has to be good in the longterm