The first game was great. It's usage of vehicles and movie like cutscenes, music, and action was all very cool, and unseen for it's time really.
However, only the first one was decent. The second one was nowhere close to anything special, just an overly shiny relatively crappy looking game that made money off the fanbase for the original.
Half-Life 2 and it's Episodes are much, MUCH better for many of the same reasons the first Halo was decent.