In previous editions it was only of use to fighter types, it explicitly disallowed the casting of extra spells. Haste is the most broken spell in the game in my opinion, because everyone has to have it. It is a huge benefit, and Mass Haste is even worse. Living City has just released a ruling that in that game, Haste provides an extra Move-Equivalent action instead, I would not be surprised to see the revised PH go that direction as well. As written it is a must-have effect at any character level. I play in a game at 20th level, and we still start every combat with Mass Haste. No spell should be able to dominate combat at every level.
In my own personal game, all Haste-like effects simply don't exist. Makes people think about tactics a lot more, particularly spellcasters.
--Seule