I was having similar thoughts about Pentax especially with the new K5, as I quite like their excellent lineup of compact primes. Historically, as you said, Pentax AF performance has struggled with C-AF and low light. That's basically the reason i have never wanted to go with Pentax thus far. The K5 is said to have addressed the AF issues with a redesigned AF module and has got me thinking about Pentax again.
That's my thinking as well. The low-light AF performance was my only real complaint with the K10D, which was a wonderful, durable tank of a body. Luckily, while I've heard little regarding the continuous AF performance, everything I've heard about the low-light performance has been very complimentary. Apparently its sensitivity is on par with the best of its competition, and, if that were true, considering the extra cross-point sensors, it would almost certainly be superior to my 5D MkII - a camera which in many ways Canon sadly crippled.
About the only thing the K5 seems to be lacking is a big enough viewfinder. But for $35, the 1.2x magnifying eyepiece accessory should sufficiently address the issue.
It's also worth mentioning that the viewfinders of the K-5 & K-7, relative to the cameras' size, are extremely large. They're no longer class-leading with respect to all their high-end APS-C competitors in an absolute sense, but relative to the size of the camera itself, and particularly the mirror box, they're unique. Apparently there was some quite remarkable engineering that went into the new mirror box and viewfinder assembly for those bodies. Pentax was quite proud of itself.
I don't think the NEX cameras will ever be good until they can get on-sensor PDAF in there. Sony has shown not only that they are inept with CDAF, but also that they are either not able or not willing to improve it to a good level like Panasonic has done with their G series (and is now even better with the GH2).
For me, with a suitably ergonomic manually focus setup (i.e. viewfinder/liveview zoom) manual focus would be how I'd like to go with these little cameras. (This is from someone who can't manually focus particularly well even with my fast primes through a full-frame viewfinder.) No motors in lenses means smaller, less obtrusive lenses, and I can move the lens' elements faster than any reasonably sized motor can.
Voigtlander has several primes that are particularly well-suited to being adapted to m4/3 or NEX cameras, and that's how I'd probably use them.
That is why i think they decided to go with the pellicle semi-reflective mirror full-time PDAF system in their SLT series to provide better AF performance with video. For my money, if the AF system cannot perform, ridiculous megapixel counts and sky high ISOs do not matter. To that end, CDAF based cameras are a non-starter for me except for Panasonic's G series.
As an aside from what I said about manual focus on these cameras immediately above, I certainly agree. I want a compact, high-quality camera mostly for social photography where its lack of intimidation factor is more key to getting candid shots than just about anything else. But a camera needs to be able to focus quickly if you're going to photography people candidly in social situations, so its certainly a deal breaker unless the interface for manual focusing enables you to focus quickly and accurately.
