I don't think this has been mentioned on this thread but don't certain airlines require you to purchase a ticket in certain fare classes that are ususally not the chapest ones in order to be able to use miles to upgrade. This can apply to both domestic -(i.e. Delta ) or International (I'm thinking CO but not sure about that)

So if you have to pay more $ to get an upgradable fare (vs. the cheapest coach fare)this needs to figure into your determination of what is worthwhile for you.

*Coach award
*FC/Business award
*More expensive coach ticket using miles to upgrade

Remember that if it's a long trip your upgraded ticket will at least earn some of those miles to replace the ones you used. I would imagine this is more of a factor for those who have lower balaces and/or less ability to earn FF miles. Those with millions of miles to burn who fly frequently would probably just redeem for F if they wanted to fly up front.