FW stability/reliability is highly dependant on the FW chipset, and many problems with FW come from insufficient voltage.
I use a small saffire for mobile recording, and it is a great soundcard. It doesn't work fine on my main station, because the motherboard sucks in all respect when it comes to firewire, even with an added FW card. This is a problem of my computer... on the 7 y/old laptop, it works as a breeze. funny, the asus FW is seen as a network ressource.... probably sharing some port with ethernet or i don't know what other weird routing. but it doesn't work better witha pci-e FW extension, unless i plug the power supply of the FW soundcard/HD..
FW is pretty good for mobile work. I didn't try to compare FW vs PCI performances though.
if i had to choose, I'd be more tempted by a RME raydat than by a FW solution though

(cause the raydat is probably the only card that can plug all its ADAT i/o into the 3 ADAT i/os of a scope pro, while leaving an extra ADAT for other stuff...) The Firefece UFX is pretty sexy, but the fact that it is firewire makes me wonder if it is really serious (is it? i have absolutely no idea... can you run 120 audio tracks on a FW interface, if yes, well , i guess it is good..).
I also went to a concert with Mr Akadin a year ago (i cite hilm so you can call hilm for proof, lol) , and we were appalled when the MAC book of the main synth guy crashed in the middle of the synth solo that everyone was waiting for the last 20 years as it was a band "revival tour"... According to the band, it is the firewire cable which was unplugged from the mac (roadies: don't walk around while your employer is doing his legendary solos, just stay in the bar with a couple of blonds will ya (lagger i mean))..., and so the guy could not use his synth. it didn't take that long to replug and get all back to work without having the band stop playing. We will never know if it is a mac crash, a firewire problem, or just some guy who forgot that usb and FW have to be firmly tapped to the KB/Computer and should have their own power supply, to avoid this kind of fun moments
