Keep in mind, the extra time and care put into making the Mega compatible with all the previous shields. Of course, that factors in with the price of the size of the board and such, plus the extra features such as the FTDI chip, auto-power sensor, a voltage regulator.. etc.
But yeah.. there are faster boards for the same price.. and cheaper.. but don't offer the support of the uber forum, and examples all over the net.
I personally got an XMOS board from a contest, which is the most recent board.. but not the most powerful, by any means. You're also able to chain them together (not that you'd ever need to) and offer 400MIPS (Million Instructions Per Second). You're able do multiple tasks at the same exact time, where as on other boards, you'd have that you know.. 1ms delay.. or 1uS delay when changing the state of the pins.
I haven't had any real use for the speed and power of it yet, but people have used it to emulate VGA signals and such.
http://letsmakerobots.com/node/10538
Sparkfun also has a board based on the same chip, but their version:
The Sparkfun version doesn't include the JTAG adapter from what I've read. (which is why it's only $50) The XK-1 comes with the JTAG adapter, and costs $100, but the JTAG adapter is actually just another XMOS chip (same version, just smaller) handles all the UART, and debugging.. which can be used as a separate chip if need be. (and skills are high enough)
All depends on what you need!
Personally.. I purchased the Seeeduino Mega, and love it! I still haven't used all the extra pins.. lol Duemilanove has worked fine for me, but I'm one of those "just because" and "just in case" buyers. 