With the NEX do you have to use the Pentax and Konica lenses in the stop down mode?
Yes, with a caveat - you can go full manual mode, stop down metering and setting your own shutter, but you can also use the camera in Aperture Priority mode, setting the aperture on the lens and letting the camera do the metering for shutter/ISO which feels pretty much no different than aperture priority on any DSLR.
I agree the best camera is the one you have with you. And with mirrorless systems, they can be carried in 'light' mode with a single lens and the overall size, weight, and volume are well below a DSLR. The physical dimensions don't even tell the whole story - it's more about volume - the mirrorless cameras overall are so much thinner throughout the body and lighter too, that even with relatively 'large' DSLR like lenses, it still feels significantly more portable and is less conspicuous. If I set my NEX on a dinner table, it doesn't take up as much space as if I were to stick my DSLR up there...so it just becomes that much easier to bring along where a DSLR wouldn't generally be considered appropriate...yet will still yield the types of results one might need in such low light situations as churches, restaurants, hotel lobbies, etc.
To be sure, I also have an ultracompact pocket cam that I bring when I know for sure the DSLR and even the NEX is inappropriate...so indeed I'm willing to compromise a lot to have a camera on me all the time, and stick with that same maxim - the best camera is the one I have on me.
As far as the limitations of the mirrorless systems - interestingly I often find myself using my NEX with the 18-55mm kit lens - which ends up being about the size and weight of a smallish superzoom or prosumer. In that mode, it's still much lighter and smaller than a DSLR, and seems to hit that same balance of compromise between size/portability and IQ/ability. The slowish kit lens is counterbalanced by the camera's ability to shoot clean up to ISO6400, which gives me more overall versaility in low light situations than I could get from a P&S, even with the slowish and compromised kit lens. Of course, it's even better if I use a nice fast prime, which I'll sometimes do, but the kit lens gives me that prosumer-style portability and the camera's outstanding sensor allow me to get DSLR image quality with it.
I used a superzoom before DSLRs - still have it, in fact. But I tend to do a lot of wildlife/bird photography, and a lot of night photography, including high ISO handheld, so my particular shooting, a DSLR is pretty much a must. I like the mirrorless because it can at least sub for the DSLR when I want to do the night and low light handheld stuff.
Sony DSLR-A580 / Sony 18-250mm / Minolta 50mm F1.7 / Sigma 30mm F1.4 / Tamron 10-24mm / Tamron 150-600mm / Tamron 90mm F2.8 macro / Minolta 300mm F4 APO
Sony A6000 / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / 10-18mm F4 / 35mm F1.8 / 16mm F2.8 / via manual adapter, lots of Pentax K mount, Konica K/AR mount, and Leica M mount manual lenses