A flesh golem has two attacks, both deal 2d8 physical damage. They are resistant to fire, cold, shock, sleep and poison, and the corpse can grant any of those when eaten. Flesh golems technically have a brain, but they do not use it and have nothing to fear from monsters who eat brains.
If you are polymorphed into a flesh golem, being hit with a bolt of lightning or any other electrical attack will heal you (the encyclopedia entry suggests that this is an allusion to the story of Frankenstein). As with other golems, turning it to stone will change it into a (live) stone golem. Flesh golems can be created by casting stone to flesh on a stone golem, which is moderately useful since flesh golems are slightly weaker, and leave an intrinsic-bearing corpse.
With an anxiety that almost amounted to agony, I collected
the instruments of life around me, that I might infuse a spark
of being into the lifeless thing that lay at my feet. It was
already one in the morning; the rain pattered dismally against
the panes, and my candle was nearly burnt out, when, by the
glimmer of the half-extinguished light, I saw the dull yellow
eye of the creature open; it breathed hard, and a convulsive
motion agitated its limbs.
How can I describe my emotions at this catastrophe, or how
delineate the wretch whom with such infinite pains and care I
had endeavoured to form? His limbs were in proportion, and I
had selected his features as beautiful. Beautiful!--Great God!
His yellow skin scarcely covered the work of muscles and
arteries beneath; his hair was of a lustrous black, and
flowing; his teeth of a pearly whiteness; but these luxuriances
only formed a more horrid contrast with his watery eyes, that
seemed almost of the same colour as the dun white sockets in
which they were set, his shrivelled complexion and straight