In a project I am using a Seeeduino v3.0 as an Arduino clone. On the seeeduino website it says that
It is based on the Duemilanove schematic, 100% compatible to its existing program, shield and IDEs.
In fact I never had any problems with shields or the IDE.
My question is now: Is the seeeduino and other clones really 100% compatible? The seeeduino board was considerable cheaper then an arduino.
Over the years I have used original Arduino boards as well as a multitude of clones of varying provenance, from the excellent InduinoX that is listed on the Arduino site as an official compatible board, to the very well-made Chinese Meduino Nano Enhancement (3.3 / 5 Volt switchable), to dirt-cheap clones from eBay that are sometimes better constructed than the official boards.
There is nothing unique or sophisticated about the Arduino board: It's basically just the microcontroller (atMEGA328, atMEGA1280, SAM3X8E or others) and its minimal support hardware. Not much that can go wrong with that little, really.
The two common elements that ensure compatibility across these boards are:
The bootloader can be replaced with a better or more up-to-date version if one is available for the microcontroller on your board.
The support files can either be found online, or minor edits to the nearest similar board's files can provide the requisite support for the Arduino environment.
In short: Yes, clones typically work just as well as the original Arduino, and clones with enhanced features often work far better while retaining compatibility (e.g. InduinoX)
If you closely look at the Arduino circuit diagram, You'll notice that it is little more than power supply, a USB-serial interface and the microcontroller itself.
The 'hard' part for compatibility is getting the board layout right (which connector goes where on the circuit board), and to load a compatible bootloader in the chip (which is free/open software, so why bother making your own?).
I've used several el-cheapo Arduino clones and apart from apparent physical construction (one feels a bit flimsy), they all work just like the 'real thing'. I guess the major difference between original and clone is the quality of the board, the used components and quality of assembly.
Check twice !
Most clones are exact clones with copies of the flaws that original Arduino board had. Like for example Arduino Nano not having proper RESET and TEST pin connection of FT232RL USB-serial bridge. This flaw may lead to FT232RL not responding to PC and was eventually fixed in later revisions of Arduino Nano board. Some clones were not updated.
Another "not exactly Arduino clones" example are numerous boards with CP2102 USB to serial bridge instead of FT232RL. This may pass unnoticed, but may give frustration when one clone board connects and works with your PC, but another generates system complains on missing drivers.
So again - be prepared !
I have found that most clones will do what you expect them to do, but as x4mer states in his answer, be prepared.
The most annoying issue, and even then it is quite minor, is that a lot of Chinese clones do not use an ATMega for the USB and instead use a CH340G. These chips, while fine when connecting to a PC running Windows 7, have great difficulties with the drivers on OS X, and end up just not being recognised. It is possible to find a driver for the CH340G, but when I installed it, it immediately crashed the MacBook, upon plugging in the cloned UNO..!
I had to buy three or four clones on eBay until I got an Uno with an actual 16u2 implementing the USB interface, which would actually be recognised by the MacBook Pro, and would allow me to code using the Arduino IDE running on OS X.
Before that, I had to use a Wintel laptop.
