Seems like there are lots of us with 3 series that are thinking about the X3, myself included. Here's what I have found out so far. The current X3 was updated for MY 2007 and changes included the N52 engine with 260 HP and a new 6 speed Steptronic tranny that has had some software issues that may have been resolved. The X3 is made by Magna Steyr in Graz, Austria. The next generation X3 will be built in SC starting with MY 2010. Build quality from the Graz plant is top notch and you can see how good here:
http://www.prnewswire.co.uk/cgi/news/release?id=173013
As with any other vehicle, you will find those that love them and would buy another and those that wish they never decided to get one. Unfortunately, there aren't as many threads about the X3 as there are on the E90's. Got to dig deeper to get the real skinny.
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2007 E92 328i 6MT-Space Gray/Black Dakota/Gray Poplar w/heated seats-PCD 4/27/07 (SOLD)
2009 E90 328i xDrive 6AT-Space Gray/Oyster Dakota/Dark Burl Walnut w/ZPP,ZCW,Xenons-ED 4/17/09 - Redelivery 5/30/09 (SOLD)
2014 E84 xDrive28i 8AT-Alpine White/Black Nevada/Silver Matte w/ZMV,ZLP,494 - DD 3/14/14 (SOLD)