I think its a worthwhile feat. Remember, it not only improves your basic hit points, it improves your healing surge value. Your functional "daily" hit points go up by 5+1.25*surges per day at heroic tier.
At level 30, a fighter will have
15+6*29+con score = 189+con. That means that Toughness counts as about a 7% hit point boost.
At level 30, a wizard will have... I forget exactly, but about 126+con score. Toughness is more than a 10% boost for him.
If you don't like it, skip it, but some people find it worthwhile.