What is e3 Feedback Implants good for?
As of right now, there are two situations in which E3 feedback implants are useful.

The first is that it isn't always 1 cred to break a subroutine. Aurora, Peacock, Battering Ram, and Wyrm all cost more than 1 cred to break a subroutine. Two of these (Aurora &amp; Peacock) are the same faction as E3 feedback implants. So it's feasible that you'd save a little bit of money over time.

The second use is against bioroid ice. Bioroid ice subroutines can be broken by spending a click. So with E3 feedback implants any bioroid ice can be bypassed by paying 1 click + 1 cred/remaining subroutine. Which makes things like Janus only cost 1 click + 3 creds.

If there are things added in the future that allow breaking subroutines via non-icebreaker methods, E3 implants would also be useful in those situations.

EDIT:
There was discussion about whether or not E3 feedback implants could be used as the triggering condition for itself, so here's a link to a thread on BoardGameGeek where this got answered. (It does trigger itself)
http://boardgamegeek.com/thread/910289/e3-feedback-implant-official-answer