I like mag. bars and have them in several personal and vehicle kits. I've not got the experience of many on this forum, but am surprised by some of the comments in this thread, many of the "problems" are easily dealt with by fairly obvious solutions.
Sparking Mag bars dull knife blade? Don't use the blade, use the spine of the blade, nice square corners on the back of the blade will generate great sparks.
Shavings too fine? cut off bigger ones. Use a saw or file of a multi-tool, or knife blade to shave big pieces. I find the saw blade is ideal, give bigger pieces and faster, too.
Wind blows them away? Put shavings on a piece of duct tape and stick it to the tinder, bark or whatever. Or put mag shavings on a bandaid from your FAK, and maybe some PJ on the cotton pad, stick it to the bark or other tinder. Duct tape and bandaids burn well, too.
Can't use it one handed? I've practised, and found ways to do so. Not easy, but can be done with effort.
I think the mag bar properly used is a great fire starting aid. As always we should carry at least three different ways to start fires, and know how to improvise others if necessary. IMHO the mag bar should be one of the three.
Just my thoughts.
"Most men take the straight and narrow. A few take the road less traveled. I chose to cut through the woods." ~Unknown~