I have the endurance, but i also have the time, and that might be the key factor. My work schedule is utterly malleable, so if i want to write, or read for 5 days straight, i can. Doesn't pay the bills, but its nice.
Just two days ago, before i canned it again, mine underlined 'weak', 'instalments', and some other grade 4 word i suddenly cant find. I forgive it the stuff like 'isn't', or don't', because i'm too lazy to use apostrophes half the time (especially in rough draft mode), but yeah, it'll underline some real head-shakers sometimes. I didn't even know about grammar-check, but i wasn't using the words wrong, so even thats a head-shaker. When i do real edits i put it back on and it does grab the odd typo, or the very occasional spelling mistake, and point out all my lost apostrophes... heh heh... I also tend to spell quite a few words purposely wrong, and it gets annoying to always see them underlined.