My wife has a 2010 FJ Cruiser that we bought new over a year ago. She has liked it a lot (she had a Nissan Armada and several 1999-2004 Grand Cherokee's before the FJ).
Here's a rundown of the good/bad I've noticed:
Good...
-They get pretty decent fuel mileage for what they are (most highway trips we get 23'ish).
-They have a good power band and the 5-speed auto seems to do a good job all around (I had a 2007 Jeep JK Unlimited and it was a disaster as far as power is concerned, we live on the bench and the JK wasn't a good vehicle to get up and down the hill in). The 2010 FJ's also received a few engine updates and have the same horsepower rating as the 2010 4-runners (I think they went from 230'ish to 270'ish). Having driven a 2007 FJ and our 2010 FJ, I didn't notice a big difference... if any.
-Our FJ has had no problems with the "fender tearing" issue (supposedly, they did a slight redesign on inner fenders of the mid-2008's... it's a relatively easy thing to spot if you're looking at used FJ's).
-We have 2 kids (5 and 7) and the back seat hasn't been an issue with their boosters (now the clamshell door design is a different story, make sure you like it before you buy one).
Bad...
-The windshields will get rock chips (I didn't notice this on my JK, and the windshield profiles seem similar?). We have 15k miles on our FJ and it has 5 rock chips, we haven't replaced it yet, but I know it's not cheap because they have to replace the white painted trim piece at the top of the windshield (my insurance will be paying for ours).
-Clamshell door design... looking back, I wish we would have bought the 2010 4-Runner, but they weren't dealing at the time and the available one's were all $40k plus.
I personally wouldn't buy a Jeep JK Unilimited until the new Pentastar motor and 5-speed auto come out in the 2012 models (again, I owned a 2007, and couldn't stand the motor/transmission combo).
If you end up with a FJ, we put Expedition One bumpers and sliders on ours and they have been great. I haven't lifted ours yet, but plan to do a slight lift and 285/70-17 tires eventually. I know Kurt at Cruiser Outfitter's has put OME suspensions and ARB bumpers on a bunch of them with great results as well.
Good luck with the purchase.