domo arigato for the software, but why do you mention Uno only? Serial communication is mostly the same for all arduino, and, if there will be native MIDI port with soft serial it will be nice for macos (and even windows) users too, isn't it?
(A) Advantage
The biggest advantage to use USB class compliant driver is the simpleness.
As FTDI bridge chip support only serial communication mode, you need to install software driver to emulate MIDI on your PC.
With replaced 8U2 firmware, you can use native class driver of your PC. Just plug and play.
The device will work with iPad / iOS4.2, as MIDI device. The device made with FTDI chip will never work with iPad.
(well, without JB, at least)
(B) Disadvantage
The software works only with Uno. So, you need to use Uno.
You need to replace 8U2 firmware with MIDI version. It is required to rewrite original firmware to reprogram m328p.
Personally, I use ICSP programmer to program 8u2 and m328p.
BTW, I don't know good serial-to-MIDI software bridge for MacOSX.
Can't we upload via the old fashioned way; a USB to TTL converter on the TX/RX?
This way, we don't need the ATmega 8U2 to convert our CDC to TTL, we'll bypass it.