I used to use USB flash with pfSense, they kept dying on me cause I cheaped out and bought crappy ones. So this proves that pfSense does a lot of small random writes that will eventually kill flash drives. However, I do use an SSD now, OCZ Synaps. An old cache drive with 50 % over provisioning. I think that is the key here. Buy a small SSD and do a huge over provisioning, maybe 20-40 % even. The Samsungs will use the unallocated space to even out the wear of the drive.
The most performance gain you will see is if you use some sort of transparent proxy with caching like snort. And you will most certainly loose some of the vibration and noise over the HDD.