Old kit lens, new kit lens or a pro zoom?

Generally the pro zooms and primes will offer better sharpness and contrast (especially towards the corners and edges)... and maybe other bells and whistles depending on the lens. The 12-40mm has some added perks like weather sealing, an excellent manual focus ring with distance scale, the ability to focus quite close, and (at least a perk for Olympus cameras) a programmable L-Fn button.

Some of the variable aperture zooms have the advantages of being smaller, lighter and less expensive. The question is how much do you give up in terms of IQ? For some not that much at all. I used to have a Panasonic 14-45mm that was probably the sharpest kit zoom I've ever owned. I put it up there with the Nikon 18-70mm f/3.5-4.5. The 14-45mm compares quite well against a lens like the 12-40mm f/2.8 or Panny 12-35mm f/2.8. In good light the pro zoom doesn't offer much advantage except for 12mm being MUCH wider than 14mm. I also had an Oly 12-50mm kit zoom that came with my E-M5. it wasn't as sharp as the Panny 14-45mm, but I ended up using it more because of it's wider wide end and it's close-focusing capability. Whatever sharpness it lacked compared to the Panny was not that obvious for most photos unless you pixel peeped. Well... it had some flaws at the corners and edges, but the center was really nice. Great for some photos, but not for others.

I shoot primes quite often... the Panasonic 20mm f/1.7 pancake is the lens that brought me to m4/3, and I still shoot with it a lot. I also shoot with the Oly 45mm f/1.8 and a Rokinon fisheye quite often. The 45mm not as much since I bought the 12-40mm f/2.8. The 12-40mm is my most used lens, but it's a beast compared to the typical m4/3 kit zoom. That said it's really light and compact compared to my old Nikon 17-55mm f/2.8 for APS-C, and it's a better lens in every regard. The 12-40mm f/2.8 is a wonderful lens, but a variable aperture kit zoom might do quite well for daytime walk-around use. I often use the slower Oly 9-18mm zoom as my walk-around go-to, but I like wide angles.

Ultimately it depends on what you shoot, where and what the lighting conditions are. Primes are great and compact, but the zooms sure are nice when I'm out with the family... or anytime when I need to shoot quickly from a variety of focal lengths. The new Oly 12-100mm f/4 sure has a lot of appeal for a general use lens as well.

Sean