Agreed with that - Valve have always been fantastic with their voice actors, and more game developers ought to follow their example. Good, natural-sounding characters voiced by professional actors makes them a ton more believable.
Wheatley's temporary placeholder voice (done by an animator) compared to the final voice is a good example of this. (Bah, embedding doesn't work for this vid)
Oh and it's interesting to note how much of a difference music and extra actions adds to a scene.
Compare that to the final version of the scene. (SMALL SPOILERS). (skip to 2:00).
BTW, I dunno why people think that Caroline is Chell's mother. There are lots of things about the Portal storyline that are open to speculation, Valve's storylines are awesome even though they're full of holes, but there isn't a lot that points to the two characters being related, not even the turret opera at the end of Portal 2, even with the lyrics "Farewell my child". You don't have to be related to someone who is much younger than you, who you're 'looking after' (or not, in GLaDOS's case - But the facility, under management by GLaDOS, was Chell's "home" for a long time) in order to refer to them as "your child".
I think people look into things too hard at times. For example in the vid Aperture Investment Opportunity #4: "Boots", Cave Johnson addresses Caroline to ask her to roll the video showing the POV of a test subject falling to their death, but for some reason people have interpreted that the test subject is Caroline, and it's a POV video of her death, even though there's nothing pointing to that and it doesn't make sense.
Cave Johnson is an evil bastard, but I don't think he's so much so that he'd send his personal secretary and perhaps the closest person to him to die in the test chambers.