I think I'll agree with the apparent consensus here that it depends. It depends a lot even on the specific car you buy, and how you buy.
Basically, if you were to buy new and keep the car then I think the odds are better that you can continue to drive the car well out of warranty. I'd definitely want to exceed the BMW recommended maintenance schedule by doing my own "in-between" oil changes (which I have done on my 135i). But even then there's a couple of caveats.
Let me use my 135i as my example; would I own this car out of warranty? At the moment I am thinking no. My car has an incredibly complex electronic system (the entire car does, not just the iDrive stuff) coupled to a relatively high-strung turbocharged engine that to-date does not really have enough "real-world" stats to say that it won't suffer some critical turbo failure at some point. It's also still a bit early to say that it will never have a problem like the N54's HPFP problem... while the stats to-date are good, it took a good couple of years for the HPFP problem to show some significant noise.
The engine is also coupled to a dual-clutch transmission. Now, I think it's a thing of beauty from a technical perspective. I think it's an incredible piece of technology that I love... but the longevity and repair costs are something I don't think there are any really good numbers on. The DCT has only been used in a few relatively low-volume model BMW's and as such there really isn't enough data to make a determination on long-term life. Now, I happen to think clutch life in this might well be better than a torque converter in an automatic simply because there's two of them and it's a consistent level of wear and tear from the system itself... but even then I wonder how much all that is going to cost to replace
when the clutches need replaced.
I have similar issues with the AWD X-Drive system (though not on my car, obviously)... it's very complex and again used on a relatively small portion of BMW's sold... so I am dubious about the longevity of the system. It's complicated compared to say Audi's Quattro and Subaru's AWD because from an engineering perspective it really is a retrofit onto a RWD car. I'd be wary of that too.
Now, having said that; if I owned a 128i, or last generation 328i or 528i with that normally aspirated I6 and a manual transmission, then I think you could say that yes I would own that car out of warranty. The engine and transmission are likely bulletproof as history has shown on earlier versions of the same hardware. Or at least as bulletproof as you can get. And I'd forego a lot of the fancy electronics... in-car electronics just "age" too fast anyway and when they break they're usually expensive to replace.
The modern x28i? Well, except the 1'er which still comes with that marvelous I6 I would probably pass; the engine is new and at least as complex as the N55 if not moreso. I'd be VERY wary of owning that beyond the warranty because there simply aren't the miles on that engine to prove it's longevity.
Even with that, there are some caveats; BMW have like all manufacturers effectively locked out the shadetree mechanic. There are fewer and fewer things you can do to repair a modern automobile yourself, and it's becoming harder and less cost-effective even for independent shops to make a go of it without investing 10's of thousands of dollars in computer equipment for each manufacturer. Even then they don't always get it right. The last car BMW made that was really "shadetree-friendly" was the E46. Even the E60 was not a good car to try to work on yourself, and as of the advent of the E90 and newer it's almost impossible to do a lot of the work yourself effectively.
Hope that all helps