They are definitely useful, although the level of utility depends on your class, skill/rune build, and the level of difficulty you are on.
In Normal, I found them to be largely useless. I think I used 4 potions before Act IV.
However, as the difficulty increases, the need for supplemental healing does, as well.
Particularly against bosses, uniques, and champion packs, the game generally rewards you for hammering on the baddies by spitting out well-timed healing globes to keep you going.
However, sometimes surviving until the next healing globe is a bit of a challenge, particularly for some of the less pleasant affix combinations. Healing potions can give you just enough health to get you to that next healing globe, which, in turn, will get you to the end of the fight.
Some escape abilities will help you prolong these tough fights, as well, allowing you to get several healing potions in over the course of the battle. As a Witch Doctor, I sometimes find myself popping a healing potion, dropping a DoT, then using Spirit Walk to buy myself some distance, nuking/dotting while kiting until my cool downs expire, at which point I can pop another healing potion.
It's not every fight (at least it shouldn't be), but they absolutely come in handy for particularly tough battles. Even if they only save your life once an hour or two of playing, it absolutely merits picking them up and carrying them. After all, they take up so very little room, and they don't sell very well. Just remember to upgrade them to the best you can carry, and don't make the mistake I did of forgetting to put the newest ones on the hotkey when you upgrade!