Here is my take on them. The mag shavings suck. As stated, you get heat out of them but it flares up in a second or so and never really seems worth the amount of time required to generate magnesium curls.
That said, the ferro rods on them are great. I also find them very easy to strike, because the mag block part produces a good hand hold. So, I use mine, but I basically use it as I would a normal fire steel, with normal firesteel tinder e.g. fatwood or PJCB or jute or charcloth/jute etc.
On the plus side. They are cheap and more easy to find in brick and mortar stores than firesteels. I can usually find them for about $5. Unlike a firesteel they don't require a handle. I find them more easy to grab the body of the block and strike compared to a firesteel. In fact, I prefer to strike a mag-block firesteel compared to a firesteel itself. I just find it intuitively easier. The little firesteel rods on the ones I've used spark great, as good as LMF rods. Also, their square shape and relative compactness makes them really easy to stow in pouch pockets like those found on sheaths.
Bads. The little rod obviously doesn't last as long as a real firesteel. However, a new mag block firesteel will last many 100's if not low 1000's of strikes if you are careful in how you strike. I'm also told that the rod will sometimes pop off the mag block due to the glue letting go. I've tried to pull the little firesteel of the magblock and couldn't do it so I don't know what to say about the last critique.
If I had my druthers, I'd have a bunch of firesteel blanks stowed away. However, magblocks are easy to find and cheap and thus I have a few of them stowed away.