I'll start by posting an Arduino clone to avoid. This one is an Arduino Mega clone with no manufacture ID other than "MADE IN CHINA" printed in the upper left hand corner of the board.
I've encountered the following problems with this board:
1) It's uses a CH340 chip as the Serial to USB driver interface which requires a driver download unlike genuine Arduino boards which will work with Windows computers without additional drivers. Under Windows 7 the CH340 driver seems to work well but I've seen many people running Windows 10 having driver problems.
2) The reset circuitry does not work. The biggest problem this causes is that if you're wanting to download your code using the USB interface you wont be able to because the firmware is not able to properly reset the board when it detects a download is being requested. A work-around I've found is to disconnect the board from USB, pre-compile your code so that it is ready to download, then reconnect the USB and immediately press CTRL-U (Upload) from the Arduino IDE.
3) Very noisy digital I/O. Every time you change the state of an I/O pin it will cause a large spike on the 5V pin. This spike will be present on every I/O interface of the board including the analog input interface. The designers of this board left off decoupling capacitors which would absorb such spikes and as a result you're left with a very noisy board which can effect any circuit that is attached.
4) The RX/TX LED's are reversed, when sending serial output the RX LED will light and visa versa.
Here is a picture of this board to avoid, it's usually the cheapest Arduino Mega board listed on Ebay. There are many people still selling them.