There are a couple forums I hang out in with other folks with ALS and their caregivers.  One of the PALS (person with ALS) posted the below in response to a question about why stem cells are more effective with heart rehab then with the things involved in ALS.  I thought it was a good overview of the challenge ALS poses. 

A PALS Post:

I am no expert but from what I have read stem cell treatment for regrowing motor neurons is still a way off. Blood cells have been replicated for years now and are effective treatment for many diseases. Heart muscle cells are also a much less complex cell and it seems there are methods developed to regrow these cells were they are needed. Stem cells have even been effective in treating damaged myelin sheath, the insulator so to speak, around axons of the neuron cells, but this is still not at all the same as regrowing neurons to control muscles all over the body.

Motor neuron cells are the cells which die off at an accelerated rate in ALS. These cells connect the central nervous system with the muscles sending electrochemical signals causing the muscles to react. These cells have a long filament like extension called an axon that allows a connection from the spinal column or the brain to the muscle. As you can imagine the axon can be quite long extending from the spinal column to the feet. As people age these cells die off naturally but many survive until one is even 100years old. The body does not regrow muscle neurons naturally. By the time a person is a year old he already has all the motor neurons he will ever have- its all down hill from there but with ALS it is much faster, like a free fall in some cases.

 Recently scientists have been able to culture neurons from skin cells. Skin cells that have that ability are called pluripotent stem cell as they have the ability to develop into various types of cells one being neurons. This has been a great step but to make these newly cultured cells work they some how have to make all the right connections when put into a persons body. Another problem is growth rate. Axons grow naturally at a rate of about 1mm per day so to develop connections at distal points could take some time. If neurons die off faster than they can be replaced this treatment will not be a cure but may slow down the progress of ALS. If a cure were found for ALS this treatment could restore lost function.

This is a very simplified explanation. There may be some tricks scientists may learn that will speed up the process such as using stem cells to repair dying neurons.

Other types of stem cell treatments may have a peripheral effect on ALS symptoms.

If I am misinformed I stand to be corrected (I am no scientist).