8 gallon carboys? Wow- they would be very big and very heavy. If you make 8 gallon batches of wine, then maybe it'd be worth it. But there is no way in hell I'd be able to lift them if they were full! I have trouble lifting full 6 gallon carboys.
I don't know how much they'd cost new, so if you've had a hankering to do 7 gallon batches of beer, maybe they'd be worth it. I'd rather get a 7.9 gallon "ale pail" with a lid for $15.