Some say fake, some say genuine. Which is it?

Here is the problem, Amazon reviews are by the product name, not seller. So you will see reviews for both fake and real. If it's coming from China, rest assured it is a fake. If it's less than ~$18, it is most likely a fake. I purchased from mp3car store for about $19 and I received a genuine Arduino. Also keep in mind there are some legitimate clones that are perfectly legal, provided they do not use the Arduino name on the board - they may may use cheaper components (e.g. voltage regulator). But these ones on Amazon for around $12 are counterfeit since they use Arduino name and trademark and attempt to trick the buyer into thinking they got the real thing.It looks like Amazon has partially fixed the problem. Previously under "more buying choices" you would see the counterfeits. The tell tale sign was they were about $13. The have removed them, but you can still find them for sale under search term "Arduino Compatible". Amazon however should ban them from sale since they use Arduino name/trademark on board.Here is an example of a counterfeit board http://www.amazon.com/Development-Microcontroller-MEGA328P-ATMEGA16U2-Arduino/dp/B00G46X31W/ref=sr_1_11?s=electronics&ie=UTF8&qid=1392856538&sr=1-11&keywords=arduino+compatible

Does it come with USB connector?

No, genuine Arduino units do not include the USB connector. Some sellers are including USB cables with their 'knock off' boards, but selling them here on Amazon is against their terms of service.

how do I attach my USB plug to this board?

You can use a Micro B cable to plug in this board.

it says have 20 I/O pin. but in picture, there are only 13, how to make use of the rest of IO?

For the Leonardo, the AI pins (A0-A5) can be used as digital pins... See the Arduino website for more details. http://arduino.cc/en/Main/arduinoBoardLeonardo

ASIN	B008A36R2Y
Are Batteries Included	No
Best Sellers Rank	#28,484 in Computers ( See Top 100 in Computers ) #300 in Single-Board Computers
Brand	Arduino
Color	Blue
Computer Memory Type	SRAM
Customer Reviews	4.6 4.6 out of 5 stars (788)
Date First Available	14 August 2012
Item Weight	25 g
Item model number	A000057
Manufacturer	Arduino
Maximum Memory Supported	2.5 KB
Memory Clock Speed	16 MHz
Memory Technology	DDR
Number of Lithium Metal Cells	1
Operating System	Arduino IDE
Processor Brand	Microchip Technology
Processor Count	1
Processor Socket	DIP
Processor Speed	1.6 GHz
Product Dimensions	7.62 x 2.54 x 5.08 cm; 25 g
RAM Size	2560 Bytes
Series	Leonardo
Voltage	5 Volts
Wireless Type	802.11b

Arduino Leonardo with Headers [A000057] - Versatile Microcontroller Board with USB HID Support, Ideal for Keyboard, Mouse, and Joystick Projects

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