Your TFSA holdings are totally tax exempt. Doesn't matter if your holdings increase by 1% or 1000%. The other side of the coin of course, if your holdings go down, you cannot claim a loss on your investments.
The feature I like about this account is , if you do hit the ten-bagger, like OZ referenced, you can pull spending money out, tax free, and at a later date you can put it back into your TFSA, once again with no tax consequences.
On top of this of course, you can add the $5000 per year. With accounts set up for families, over a few years this could develop into serious money. I'm not sure how long our lying government ( remember the Income Trust debacle) will let this run, but the way it's being presented, it's a 'must have'.
The problem I'm having at the moment is getting the bank to set up our TFSA's as trading accounts (like self directed RRSP's etc.). Right now they're only setting up savings accounts, emphasis on 'savings' as in Tax Free SAVINGS accounts. On these of course you'll earn less than the inflation rate.
Hope this helps, and if anyone can detect errors, please jump in. That's why I posted this.