Depending on the GPU heatsink you can find, they can be worthwhile - if you plan on using the card for at least another year. 3rd party solutions can help extend the life of the card, esp if you wish to significantly overclock it. I swapped out mine on my 6600GT for a zalman solution and actually decreased its load temperature by 15 degrees, and thus allowed me to push the cards capabilities.
Further, 3rd party designs can be be big enough and/or better than the original (mainly cuz the solutions used are not as efficient - the companies need to make money somewhere right?) so that you can employ a fanless design (if you plan on not using it for gaming in the future), or one with speed control.
It all depends on how long you wish to keep using it and whether you can get a good deal, imo.