We use buckets like these in all our stalls during the winter. I have never had a horse refuse to drink from them. Actually they tend to drink more water out of these heated buckets then normal buckets you remove ice from. We typically have to fill them 3 times a day for a stalled horse.
On a side note, we had one gelding boarded with us last winter who would back up to his bucket and sleep with it against his cheeks. Of course he would then poop in it sometime during the day or night and the results is not pretty or fun to clean up!