3. Femme Fatale is extremely expensive, is not a particularly good sentry breaker, and is only really effective against one ICE for the entire game. Unless that ICE happens to be Janus, Heimdall, or something along those lines, it's not overwhelmingly a good deal for you. Unless you also use her to handle low sentries (particularly rototurret, which is a valid use of her), I don't think she's worth the space in the deck. A possible strategy is to use Femme for low strength sentries and ninja for high strength sentries. This is best done in Criminal, however. That said, many of the better players I know like Femme and continue to use her. She doesn't fit what I like to do, and so I keep her out. If better players than me like her and keep her in their decks, they must see something I don't.
At the moment the best ice in the game is Tollbooth.
And Femme is the cheapest way to get around that, you bypass it for ONE and don't even need to pay the fee.
When I started Netrunner I didn't like the Femme at all, but the longer you play the more you might like it.
Femme gets you around Data Raven, Toolboth for very low cost. (no need to remove Tag or pay money)
Even against Ice like Archer Femme is very good, bypassing Archer for only 4. (10 with Ninja)
If you want to run a specific server very often and it is blocked by some annoying ice Femme is the way to go.