A few days ago I tried serial to MIDI from this tutorial: http://www.spikenzielabs.com/SpikenzieLabs/Serial_MIDI.html
and now I want to share my experience with it.
I used it to send MIDI from my Arduino UNO to FL Studio on a Win7 64 bit PC.
1. Setup: That was pretty simple. The MidiYoke setup crashed when I started it the first time, after a quick search I found out that i has to be installed in 32 bit compatibility mode. After a restart, setting the ports and restarting again it was working.
2. Using the Serial-Midi converter: Being written in processing it just needs to be started and works "out of the box" especially after carefully reading the instructions
3. General usability: It's not using the standard MIDI baud rate, so you have to make some changes in the MIDI library (if you're using it). Every time you want to upload a sketch, you have to quit the serial-Midi converter and restart it afterwards, which takes a lot of time when done often. Also FL Studio was kind of infested with bugs thanks to MidiYoke
(could be because it is not written for 64 bit cpus, still strange).
4. Summary: It's okay for playing around when you want to try MIDI before buying specific hardware. But dont expect it to be a perfect substitute for "hardware"-MIDI. Also I think that absolute beginners would despair completely, so its more suitable for users with a little experience in Arduino. If you're going for a more serious project, get some MIDI-hardware (I did), it is a lot more comfortable and most important, you don't have to go trough a long journey of adjusting settings when you're using it on another computer.
Feel free to share your experiences if you are using or were using it.