I like Karen just fine, and think Matt does a great job.
As for her leaving, I guess companions come and go. I like her character progression, but like most companions they seem to graduate from a meek "help me, help me Doctor" to someone who can really hold their own (even Rose became all badass when we last saw her).
I was a newcomer to Doctor Who when season 5 began (Matt Smith's first year), and I watched those as they came out, and then season 6.
THEN I found (what a great deal) the complete season 1 - 6 dvds (that's right, all Doctor Who since the 2005 restart) for only $65.00.
I just finished season 4 (the last David Tennant season) to the episode where he regenerated as Matt Smith (so I'm basically all caught up).
So Matt and Amy were my real first Doctor and Companion, but I really do like Rose. Rose or Amy, I can't decide which is my favorite (I'll give Amy a slight edge, why I don't know. Red hair maybe? hehe). Maybe its a "first love" syndrome (first "Doctor"

.
I like Matt Smith and David Tennant just fine, Christopher Eccleston not so much.
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http://www.youtube.com/watch?v=BRdHJAX4S_w
^^ Bloody Blackberry (original song/video)
http://www.youtube.com/watch?v=q7khBUbMHVQ
^^^ "She Used to be a MILF, Now She's a Zombie" (my best original Youtube vid. Song was fun to make)