[William]

Greg Raymer (fossilman) wrote about add-ons some weeks ago. i don't know how to create a link to his post, but I have copyied it below, he clearly explains the why's and how's.
As far as the rebuys, I certainly would prefer joining an ongoing tournament than starting in a new one.

Greg's post:
Here's my method.

If the add-on gives you more chips per dollar, always take it (simple enough, and I doubt we'll see any controversy on this statement).

If the add-on is the same price per chip as the original buyins and rebuys, take it if you're an above average player, and skip it if you're below average.

If you see Chris Ferguson with a T1000 stack, and the average value of every chip in the tournament is $1/T1, you would probably say that Chris's expectation in that event is greater than $1000. As such, each chip in his stack is worth MORE than $1/T1, maybe significantly more.

On the other hand, if some newbie is sitting next to Chris with the same stack, you'd have to say his expectation in that tournament is a LOT less than $1000, so his chips are worth less than $1/T1.

If there is an addon available, $1000 for T1000, Chris will spend $1000 to get chips worth something more than $1000 in his stack. The newbie will spend $1000 to get chips worth LESS than $1000 for his stack.

Solely considering today's monetary EV, Chris should addon, and the newbie should not.

Later, Greg Raymer (FossilMan)