I had bitp.it running on a fairly high-traffic (non-bitcoin-related) website and didn't get anything from it at all.
Think it works better the longer you stay on the site. That's already the first problem as most users usually leave websites after some minutes.
bitp.it DOES work but you really need high traffic sites. preferably blogs/forum where users stay for a while.
I've tried it too and had ~100 workers >constantly< mining for 12h and generated 2 shares.
The hashrate was 80kHash/s to 180kHash/s the whole time. So you really need high traffic sites for this to work. A 100visitors per day won't get you far. You will need +1.000 visitor at any given time to get +1MHash/s and even with a thousand permanent workers it will still be more than 200 times slower than a single up to date gpu.
bitp.it can be considered as experimental for now but i think it will be really interesting when webCL rolls out or someone finds a way to implement sha256 in webGL