Need info regarding this:

1> on eBay for $19 ( http://www.ebay.com/itm/Optical-Flow-Sensor-APM2-5-improve-position-hold-accuracy-Multicopter-ADNS-3080-/310702206281?pt=LH_DefaultDomain_0&hash=item48574b6d49 )
2> Is it plug and play, for APM2.8 ? ?
3> Need for indoor accuracy, will do its job ?
4> Between GPS and optical flow, when outside how will it help if i already have the GPS ?

Why do you have to keep opening new threads for subject related to APM? You can document it in a single build thread so that others can find all related info at one place.

Anyways, the module is old, not very well supported for further enhancements I guess. Its not very accurate although it is helpful in a non gps environment to hold position to a great extent in ideal conditions rather than having no module at all. Im not sure but range might be limited to 10-20 meters. Not too good for indoor accuracy due to lighting/flooring etc. It depends on where you are trying to fly.

While the new module wont work with APM 2.xx, but neither it is too useful for pixhawk now as
"The PX4Flow is not supported in the current stable releases of ArduCopter, ArduPlane or ArduRover software. Support will be added for the Pixhawk in ArduCopter 3.3 but it is unlikely that it will ever be supported for use with the APM2."

http://copter.ardupilot.com/wiki/common-optional-hardware/common-optical-flow-sensors-landingpage/common-px4flow-overview/

Some data that I found about setup can be viewed

http://copter.ardupilot.com/wiki/common-optional-hardware/common-optical-flow-sensors-landingpage/common-mouse-based-optical-flow-sensor-adns3080/

http://diydrones.com/profiles/blogs/adns3080-optical-flow-sensor-now-available-in-the-diydrones-store?id=705844%3ABlogPost%3A713013&page=2#comments

I thought opening a different topic for different question is better ( was initially planning to ask it in the same thread )

MODS can move it the APM thread.

ok, So you suggest against it ?

reading the docs and stuff is fine and I've already done it , but i want an answer from someone on ground zero, that is the reason i posted it....

My first suggestion would be not to fly indoors. Moreover this module has issues with flying indoors due to lighting and in some cases flooring as it is. Developers themselves state that this sensor isnt apt for indoor flight. If you had read the docs etc or the links posted earlier then you would find that all your questions are answered by the developers themselves, who are at the 'ground zero' if I may say.

The first link previously posted about setup answers your ques 2.
As you can read in arducopter wiki, its not plug and play; moreover nothing related to APM is plug and play at all!
You would have to rewire it , properly connect it and solder and cut some pads underneath the FC. You have to check  it by loading a test sketch via arduino IDE or MP and then calibrate the lens by capturing an image by connecting the senor to PC via python Then I believe you can change parameters on how do you want to use that sensor in case of a gps fail safe scenario. I believe you can instruct the autopilot on how much to use data from that sensor etc. And all APM's from 2.0 to 2.8 are almost similar and since yours is the latest one it will work. Connections may/may-not have changed so you would have to look at the schematic of your particular FC and match it with the instructions.

The second link is a discussion where the arducopter dev Randy himself is discussing about the sensor and how it works and stuff, where you would find answers to question 3/4 and even 2. You can even ask questions there if you want so that you can get answers straight from ground zero 

Got it. Thanks for clearing things out