I am.....?

As I sit here with my new laptop/notebook/thingy 'facebooking away' in one of my favourite neighborhood coffee shops between classes... "Countless" other things I could be doing of course-haha.) I came across this again..

I am your constant companion.
I am your greatest helper or your heaviest burden.
I will push you onward or drag you down to failure.
I am completely at your command.
Half the things you do, you might just as well turn over to me,
and I will be able to do them quickly and correctly.
I am easily managed; you must merely be firm with me.
Show me exactly how you want something done, and after a few lessons I will do it automatically.

I am the servant of all great men.
And, alas, of all failures as well.
Those who are great, I have made great.
Those who are failures, I have made failures.
I am not a machine, though I work with all the precision of a machine.
Plus, the intelligence of a man.
You may run me for profit, or run me for ruin; it makes no difference to me.
Take me, train me, be firm with me and I will put the world at your feet.
Be easy with me, and I will destroy you.
Who am I?

(psst...I am a habit)