The guide to mag blocks from Dougs Gear section is copied below. Yes I know it's sad how we all keep asking questions when we should simply look at the 'FAQ's. I wrote a quiz for one newsgroup, based on the FAQ's and one chap was amazed how much was in the FAQ's; and he'd written then!
Simon

The magnesium block by Doan (1 x 3 x 3/8 in.) with an integral 3/16 in. flint rod glued on top (also manufactured for other companies and for the military) is still popular, at least in part because it is widely available at the consumer level. Unfortunately, the magnesium is not necessarily as effective a tinder as you may be led to believe. While the flame from the magnesium shavings is extremely hot, this flame is also relatively short lived. Additionally, the magnesium scraping/shavings can be difficult to use as tinder in some circumstances because they are so light they tend to blow away with the slightest breeze. Moreover, scraping together a small pile can be difficult at times. So, the bottom line is that they work, but the magnesium tinder has some limitations that must be understood and dealt with.

Other fire starters incorporating flint and magnesium are made by Mag/Flint Firestarters, World Survival Institute and others.

The World Survival Institute (WSI) fire starters include slim rods of flint and magnesium glued end to end on top of a length of hardwood and come in a variety of sizes, all relatively small. A leather thong a hardened steep scraper/striker (a piece of a hacksaw blade). The wood can also serve as tinder. This is a more versatile design, but the wood must be kept dry to be of much use as tinder. While the flints are smaller, they will still give you thousands of strikes. There is a lot less magnesium than the block style offers, but it will do for plenty of fires, certainly enough for any typical survival situation.

Mag/Flint and others sell flint and magnesium fire starters with large rods of magnesium and a small flint attached with a handle affixed to the end, usually made of antler or wood. These are often seen at fairs and primitive survival gatherings. With the exception of the fancy handle, they are no different than the block style, with all the same advantages and disadvantages. The handle can make it easier to work with, but the difference isn't all that great. The handle tends to raise the price and size quite a bit.