About as good as it is long.
...does a better job of hiding its linearity than Half-Life 2
Then again, it might just be because it's so short.
Episode One is mostly worst parts of Half-Life 2 mixed with some really really bad design decisions. Such as: Focusing the game on Gordon/Alyx relationship while Gordon is still a mute; Alyx doing everything for the player through first half of the game, then turning into useless groupie in second half; Putting player in dark, underground areas with flashlight lasting ten seconds; Having the obligatory "defend for x minutes form infinitely respawning enemies" section in said area; Combining grenade-wielding zombies with an NPC which blocks your way and some tight corridors; Using scripts to make things dumber, not smarter, such as reprogrammed rollermines automatically exploding exactly when when all enemies are dead and Combine soldiers charging out of cover straight at sniper positions. The story doesn't make any sense at all - Alyx uses her gizmo to mysteriously "zap" various things, which ends up with her getting some super sensitive Combine data. So the only logical thing to do is run around all of the City 17 *drawing fire* so that a dozen civilians can escape - escort missions, who doesn't love them. The game is predictable as hell - with only Gravity Gun in the Citadel we don't meet any enemies at all, but as soon as it's upgraded to being combat-worthy the levels swarm with Combine soldiers who never seem to drop their weapons because hey, it's The Blue Gravity Gun Section, and all the physics puzzles are same as first time around, and so on, and so on.
The Bottom Line
That's one afternoon you will never, ever get back.