Take a look for yourself: http://jsperf.com/bitwise-compares
On my iPhone 4, using bitwise is a staggering 60 times faster, and on the high-end Samsung Galaxy s3, it's over 70 times faster.
is the same as
123.123 << 0
can be done like this
(0.5+123.123) << 0
Now go out there and increase your app's performance tenfold!