News about the F train express, which would offer faster service between Church Avenue and Jay Street-Metrotech, is tearing Brooklyn apart, pitting neighborhoods to the north like Carroll Gardens, Cobble Hill, Kensington and Gowanus against south Brooklyn, which would reap most of the benefits (7th Avenue in Park Slope would be included in the express stops). Now that the MTA released its report (PDF) on the F train express—here are the main takeaways:
The F express would benefit southern Brooklyn communities the most while neighborhoods on the local line would have longer waits. The MTA says, "Local stations between Church Av and Jay St-MetroTech would have 50% less peak F service." Also: "There would also be increased stair crowding at a couple of local stations due to larger PM exit surges per train."

Sorry, Bergen Street. (MTA)
The most time a rider would save on an express train is 7.3 minutes. The average is half that: "Express riders during the AM peak hour would save on average 3.4 minutes and local riders would lose on average 1.3 minutes, for a net travel time benefit of 27,000 minutes." The MTA offered some scenarios of F train rides for users starting in Brooklyn:

(MTA)
Overall, the minutes "saved" by south Brooklyn outweighs the minutes "lost" by local riders. This chart lays it out—there are slightly more "inconvenienced" (i.e. local riders) but total minutes saved by express stop riders add up.
The F express will bypass a huge chunk of riders. (Duh.) The express service is supposed to alleviate the overcrowded and delayed service for those in south Brooklyn. Splitting the line into local and express service will (ideally) alleviate overcrowding, but a huge portion of riders live at Fort Hamilton Parkway, 15th-Prospect Park, 4th Avenue, Smith-9th Street, Carroll Street and Bergen Street. This chart (below) shows typical weekday peak morning commute ridership for 2014—by the time the train goes express at Church, it has 5,870 riders; the local stops between Church and Jay Street-Metrotech total 7,950 riders.
Opening up the Bergen Street station's lower level for F train express would cost $75 million. The MTA says, "Conceptually it would be possible to restore the lower level and allow express trains to stop there, thus mostly eliminating the negative impacts at one of the most heavily used local stations. However, restoring the lower level for use would require significant and costly reconstruction, including" making it ADA-compliant; reconstructing platform stairs; replacing all the architectural finishes; and restoring staircases, among other things.
The report claims, "This work is estimated to cost in excess of $75 million." That's a big number, but consider the cost of the mayor's planned BQX streetcar: $2.5 billion.
No more trains can be added to the rush hour service because there are no more trains and no more room. For starters, there aren't any more physical trains that the MTA can add, and the next fleet of trains will only come in future years. By then, they might be able to add one more train per hour, but that would be the maximum of 15 trains per hour. (Currently, the morning rush has 14 trains/hour and the evening rush has 12 trains/hour.)
MTA would only recommend F train express to start in Fall 2017, after the Culver stations project. So this could happen, for real.