Nick Devlin says of the Sony A99:I tend to agree: previous Sony "SLT" cameras kept the mirror and its extra depth requirements for the sake of supporting PDAF, but now that Sony has that on the main sensor, it seems redundant. Maybe Sony is not yet confident that its in-sensor AF is as good as its traditional AF, but at most that seems a transitional "belt and suspenders" cautiousness.
So far, none of the on-sensor PDAF systems are anywhere near as good as traditional PDAF. Nikon's seems to be the best in bright light, but I suspect it benefits greatly from the increased DOF of the small sensor / short focal length lenses the system uses. In dim light, it shuts down completely and the camera relies only on CDAF. Canon's version, on both the T4i and EOS M seems pretty dismal. Sony's "belt and suspenders" approach leads me to believe that their version of on-chip PDAF doesn't solve the problem either.
As for the question of OVF vs. EVF, like most things in life, it's a matter of trade-offs. In good light, the OVF of a FF camera is much easier to view and compose on than today's best EVFs. And in theory, you could still do overlays of shooting information, histogram, etc. In dim light, though, the light amplification capability of an EVF can be a godsend, especially for those of us with aging eyesight. And EVFs offer the overlay of shooting data, histograms, overexposure / underexposure blinkies and other information today. EVFs also offer 100% coverage, which most OVFs don't. Oh, and no need for a eyepiece shutter when shooting on a tripod.
Step down from FF cameras, and the advantages of OVFs weaken a little. Step down to entry level DSLRs with their tiny, dim penta-mirror designs, and I'd give the advantage to the best of today's EVFs.
As others have pointed out, EVFs offer real advantages for manual focus and the ability to preview the actual exposure. OVF's are still superior for tracking rapid movement and high-speed burst shooting. But EVFs continue to improve. OVF's are pretty much as good as they're going to get. (Actually, in some ways they've regressed. The OVF on my old 35mm film cameras were better in many respects than on today's APS-C DSLRs. More contrasty for better MF, and useful DOF preview, and higher magnification.
Shooting sports or wildlife, give me a fast working DSLR with an good OVF. Shooting in dim light, I prefer an EVF. Trade-offs--no one's made the perfect camera yet.
The bottom line is -- the SLT camera (and pure mirrorless as well) has some really important advantages, but it requires an EVF for framing. So if someone cannot stand to work with an EVF, then they will not like the new Sony Alphas.
Unfortunately, there's a downside, too. (I like mirrorless, btw, and use a GH2 pretty extensively.) Since the shutter has to be open for viewing on the EVF, there's a double motion to take a photo: the shutter needs to close first, and then begin the exposure. Not only does this increase shutter lag, it seems to cause more vibration. There's a fair bit of empirical evidence that at certain shutter speeds increased camera shake is a problem, especially with some of the lighter bodies.