Like some one says here, 'hand holdable' is relative. This kind of lens would be for inaccessible wildlife - and may be 'arty/graphic' nature shots!
Hand holdable does not constitute a ramble down Main St or a stroll in the local park!
It's hand holdable if you can carry it for several hours without busting a disc, climb to 5000 metres into the high Himalayas after ibex or snow leopards and come back with a few sharp pictures exposed at max 400ASA, not 64,600 ASA or whatever!!
Carrying an 800 mil lens around is a matter of size/weight/stamina. Taking sharp pictures with it at low ISO speeds is more a matter of technique/practice.
I do not own a 800mm lens but I do own a 7.62mm FN Mk1. Weighs about the same, and although better balanced, is a pain to carry in rough terrain and difficult to be shoot with without first catching your breadth. Anybody planning to use the 800 mil bazooka hand held would do well to train first as a sniper!