A solid game that can keep you entertained for days.
It may not be the best looking game (though it's far from bad looking too) and it does suffer from some minor glitches, like the mouse cursor going funky on ya at the start of each mission (which there is actually a fix for), but the overall gameplay is fantastic!
I've also heard a lot of complaints about the AI being silly, though, this is not as common as critics make you believe. I've been playing a stealthy character mostly and only occasionally some enemy wasn't reacting while shooting at it. I've noticed it only about 4 times throughout my entire first run. Most of the time the enemy AI is reacting as one expects within a game.
There is one negative point I have with Alpha Protocol however.. The boss fights. Some of the bosses are just ridiculously hard and can't be fought creatively. They use a fixed strategy that you have to adapt to - negating much of your (stealth) skills. They seem too much geared towards plain assault characters, IMO.
This is also the sole reason why I put "hard" under difficulty in case someone was wondering. Otherwise the game would have had a difficulty of just right.
Other fun things worth mentioning are: the characters you meet and interact with, the branching of the story, the customization of your weapons and armor, the e-mail and intel system, the mission variation and the upgrading of your characters skills/perks.
If you're a RPG fan that enjoys tactics and shooters you should definitely give this one a try.
Also, if you happen to be familiar with the old Project IGI game.. This is like an RPG version of that. Just thought I'd throw that in too. :)