What makes a good Showreel?
 

The title says it all, what makes a good showreel.
I'm in between jobs right now and looking at getting a new showreel together. I have been looking around the net and seen some great ones with some outstanding content but if your someone like me who freelances a lot as camera op on live gigs I don't have a lot of footage to access.

So using what I have, how long should a showreel be?
1,2,3 minutes?
I know a showreel might depend on what job you are going for but what about getting a generic reel together showing what you are capable of.
What should one include in a reel.

Cheers