yes
I had a crap flat chest
did push ups for about 2 weeks, and grew a chest
lines in the middle are apparent, and mass is growing, they work, i'm no fitness guru, but:
do them slowly. the more you do, the body just, well lets say you won't benefit, apart from in strength
secret of building mass is to do low repitions, and do them slowly. I do just three sets of ten push ups, very slowly, every day. although really you should do this every other day, fast is just for stamina, same goes with weights. heavy weights, low reps = mass, lighter weights, fast reps = stamina and possibly more weight loss
drink the 8 glasses of water a day too (recomended) as this will help you out a bit
if you raise your legs up so say, do the push ups with your feet perched up on a sofa, this'll put more strain on your push ups and make them harder, and give you more mass from doing a more strenuous exercise, it's not much harder really, unless you're a big guy. this will speed things up a lot. have your arms set shoulder width apart, and try doing them with your arms closer, and further apart to work other areas of your chest, worry about that after a few months or so, after you've seen some changes
take a look at your chest, do them for a couple of weeks and see

bet you'll see some improvements. ladies who used to know me and my flat chest have noticed a difference. cheesey but sh*t, i'm a cheesey man
also, change your posture, stick your chest out just a bit for a while (just a little) that'll help firm up your chest, make you feel better, and improve your posture. doing this while you're doing your work outs feels good, as silly as this sounds. well with me anyway. you'll feel more athletic like you're on a good path. oh and stick them out a bit during exercise and generally, don't slouch. sticking them out while doing your push ups, if you can manage it puts focus on your chest when you're working out, has more impact on your chest
again i'm not a fitness instructor so hope nobody slates me, but from a bit of research i'm quite sure all the above is true, interested to see what the pros think of this
good luck !