The Mega 2560 comes in their new (as of the release of the Uno, previously just a static bag) simple packaging: nicely printed card-stock box folded around the board inside an ant-static bag. Upon opening the box you find an insert that thanks the user for supporting Arduino (see picture below). It provides information about how/where the Arduino is built and a warranty statement. Also included is a small sheet of stickers if you want to advertise your project as having "Arduino Inside." Since there are no jumpers to set or any switches on the board, the next step is to just plug it in.
The Mega 2560 is significantly larger than its little brother the Uno (4" x 2.1" versus 2.7" x 2.1", ~50% larger), but in this case Mega is a relative term. It is still smaller than an iPhone 4 (4.5" x 2.31"), but it would only be appropriate for projects where you have a lot of room or where you need a lot of I/O. The Mega has 54 digital I/O pins (14 which support PWM using the Atmel hardware timers), compared to 14 (6 PWM) on the Uno. You get more of just about everything, actually - here's a little table for comparison:
Besides that, all the other specs are the same (voltage, current, USB interface, etc.). The pin numbers and corresponding pin functions from the Uno are mostly the same/in the same place on the Mega 2560, however some things have been moved around (notably I2C). This spreadsheet does an awesome job of summarizing all the pins and functions between the Uno and the Mega 2560.
There are a lot of pins! The Mega 2560 may only be 50% larger in size than the Uno but it has almost 4 times the I/O, thanks mainly to the large double row at the back. It's like an stretch limousine version of an Uno. I like how the pins are grouped by special function - PWM, Communication, Digital, Analog, Power - and that the Arduino Team did their best to keep things in the same place so that shields made for the smaller boards would still work on the Mega. Speaking of shields, someone needs to make a mini shield to access the back portion of the PCB that extends beyond where normal shields stop. I could see it matching up with the outline that most shields have (to access the ISP pins) like jigsaw puzzle pieces.
With its vast array of I/O and additional memory space, the Mega could be very useful for some projects, such as robotics, LED arrays, or sensor interfaces. However, for those just starting out with microcontrollers or Arduino, it's probably overkill. The retail price difference, $50-$65 for the Mega versus $30 for the Uno, would also sway people towards the Uno and limit the usage of the Mega to projects that really need the added features. Below I have listed some additional thoughts about the Mega:
- Big - lots of I/O for projects that need it
- Memory - if your code simply won't fit in the Uno, you've got much more room here
- Same/Similar shape/layout - many shields made for the smaller boards will still work
- Nicely organized - I/O grouped by function
- Big - not for your space-constrained project
- Price - about double the price of the Uno
- Shield Compatibility - limited support for large I/O expansion header