I'm a bit of a BMW fan since getting my first in 2009. We've temporarily got 4 BMW's in the household, a 2009 E90 that's about to go back to BMW, a 2007 Z4MC, the GF's 2008 120d Coupe and (wait for it) my newly acquired run around, a 2004 X3 3.0i auto (there, I said it). I also had a 1998 Z3MR last year.
Build quality opinions are often based on the bits you can see and hear that aren't right, not the rest of the car doing its job. After totally re-building the suspension and brakes of the 13 year old Z3MR I have to say I was pretty impressed with the quality of the components. Brake lines all intact, diff / driveshafts were solid, exhausts perfect, no bodywork rust, everything electrical working fine, etc. And everything comes apart and goes back together nicely on a BMW, compared to when I built my kit car and used Ford bits. Having said that, the reason for the rebuild was that the rear diff can tear the subframe on the Z3, mine was ok but I seam welded it anyway.
Interior wise, the E90 was the best quality but had creaky door seals and a rattly dash. The 120 is a cheaper interior, although it's still pretty well put together. My Z4 feels very tight and rattle free but it's only done 23k miles. The X3 has done 60k and looks dated, and it has the most rattles and creaks, although not too bad. So for me, I give a thumbs up to BMW quality, but I don't think it's any better than my recent VW's (better designed, not better put together).
Re warranty, I got the X3 from a main dealer last month as I wanted a warranty on an 8 year old car. Took it in after 2 weeks to have what I suspected was a loose wheel bearing tightened. They replaced both front discs & pads, and one caliper (coz it was sticking - yeah right!). Don't for one minute think that it was needed, but I got new brakes out of the warranty so I'm happy.
For me with the Z4MC, it's all about the driving experience and the noise. I love the steering on turn in, love the engine noise, the style and the single mindedness of the design. Fingers crossed nowt goes wrong, I dont have a warranty on the zed.