I think MtGox charges too much, but Bitfloor is a good example of a fair fee system. They actually have a negative fee if your offer is not immediately accepted, which inventivizes offers not likely to be accepted right away so people get a better picture of the market.
Thanks for the reference. I didn't know that they actually pay you for executed limit orders. Intersango also seems to have a 0.6% difference between immediate and limit orders (
https://intersango.com/fees.php). Perhaps it's a good thing to incentivize the liquidity providers.