As soon as the healing is complete - the scar tissue, the nerves settling in, and just the general settling in, they feel just like yours. I never felt like I had a foreign anything on my body. Ever. For the several months I couldn't sleep on my stomach which was hard because I was a stomach sleeper. Then I developed this way to sleep w/ my chest propped up on my hands, sleeping on my stomach - which managed to hyperextend my back. Had to stop that. It wasnt' until about a year ago I was able to get a massage laying flat on my chest w/o a towel rolled up to prop up my chest a bit. The only issues I have now are needing a couple layers of good sports bras if I plan to run at all (e.g. treadmill) and the girls tend to get in the way when I'm doign a cable row or some sort of crunch or some such yoga / pilates somethign or other.
The only other time I really am aware of them is when I diet down for a competition - around 10% bf I can start to see the rippling of them, but since that's not really a bodyfat I maintain, I don't worry about it too much.
Oh yea, if you do a serious chest flex or something like a cable fly, you do see that the implants get stretched a bit - like a balloon being pressed against.
I still dont' consider that a big deal relative to the good stuff about having them.