Let's jump to the conclusion first: This lens is a very good buy at $200. This lens can not replace the Sony E 24mm ƒ/1.8 ZA, but, comes very close to its performance in many ways. It even bests it in size and sharpness.
The Details
I was able to get out and shoot a number of comparisons between the two lenses. I was motivated to buy and compare the 30mm to the 24mm because of some lab results from lensrentals.com. Essentially, they found the Sigma to be very sharp, even wide open. But, let's start there. "Wide open" is relative. For a prime lens, ƒ/2.8 is not particularly large of an aperture. It's about a stop larger than the kit lens at the same focal length. That can be the difference between 1600 or 800 ISO. It can be meaningful. It won't, however, give you eye-grabbingly shallow depth of field for normally distant subjects.
Sharpness
It's very sharp, especially at ƒ/5.6. Both lenses peak at ƒ/5.6. The Sigma's sharpness is dulled somewhat by less than excellent contrast. It's not bad, by any means, but it's no Zeiss. It also doesn't handle bright light sources nearly as well as the Zeiss, likely due to differences in coatings. In the dark, lights such as headlamps on cars generate huge halos and they are quite unattractive. However, in more normal lighting scenarios, you won't notice these issues. I noticed it mostly on buildings with the evening sun shining bright on the sides. The Zeiss held the details far better in those areas than the Sigma. But make no mistake, under slightly more controlled lighting, the Sigma is definitely as sharp or sharper than the Zeiss at normal to long distances. The Zeiss does better close up, which brings me to…
Close-ups
Macro. Reproduction ratio. The Zeiss has about double the reproduction ratio as the Sigma. There is absolutely no contest here. I can shove the Zeiss several inches away from the face of a subject and show you the bacteria crawling around in the oil in the pores of their skin. (Okay, maybe that's a slight exaggeration but it's amazing. The Zeiss is very well corrected for close focus.) Even though the Sigma is a longer focal length, I can not fill the frame with as small of things. In fact, the "macro" capability of the Sigma isn't even worth mentioning. It has nothing even close to macro capability. The Zeiss, with its sharp close range correction coupled with the 24mp sensor of the NEX-7 means I can shoot very close up on a whim, crop it, and you wouldn't even know I didn't have a macro lens with me. Versatility is where the Zeiss comes out clearly ahead.
Bokeh
The Zeiss has better bokeh in most every situation. Not by a huge margin, most of the time. Sometimes the Sigma has this split bokeh appearance, which has some Japanese
In fact, that is a good summation of the Sigma, it performs like a great ƒ/2.8 zoom except it doesn't zoom.
Autofocus
The Sigma focuses a little more slowly. I suspect this is due to its odd design. It seems to use magnets for focus, or the gearing is extremely loose without power because the entire focusing assembly slides freely when the lens is off the camera or the camera is off. A magnetically controlled focus assembly would be a cool way to keep size down.
Size
The Sigma is smaller, significantly. However, it's no pancake. It sticks out beyond the hand grip on the NEX-7. It is not flared towards the end like some pictures make it look, it's a straight tube whose diameter is about half a millimeter less than the widest part of the metal lens mount on the NEX-7. Oh and the Sigma has a metal mount, not cheap plastic like the low end Sony alpha lenses.
Manual focus
As with all native E mount lenses, it focuses by wire. This is fine, once you get used to it. The faster you turn the ring, the fast the focus changes with a curved acceleration rate, which is nice once you get the feel for the rate. Specifically with the Sigma, however, there is a small problem. This is where the cheapness shows most clearly. If you apply even a small amount of pressure to the ring, which you are lightly to do out of habit of holding your camera steady, you will deform the ring slightly into the lens body and "brake" the ring against the internals of the lens. When this happens, it is difficult or impossible to turn the ring. You must hold the ring lightly and then it turns easily. This is something you can deal with consciously, but, may bite you in a stressful shooting moment.
Accessories
Sigma thought it necessary to provide a thickly padded zipper case for the lens. However, they thought it unnecessary to provide any hood of any kind. The lens element is almost flush with the lens front. It's small and slightly bulbous exactly like the Sony E 16mm ƒ/2.8. Both lenses are quite vulnerable to a scrape on the lens. The Sigma, however, seems fairly resistant to flare. I had no issues with flare and could not force it to occur when I tried. Perhaps on a day with fewer clouds and earlier when the sun is brighter I could get some blobs. I wouldn't worry about that. I would worry a little about protecting the front element. But here's the rub, if you buy and attach a filter for protection, the filter will undoubtedly flare like the cheap garbage most filters are made out of. Since this lens is so cheap and you'll probably shoot at ƒ/2.8 - ƒ/4 99% of the time, I wouldn't put anything on it. Just accept you might get a scratch or two on the front of your ridiculously cheap lens. No big deal. Finally, unlike Tokina lenses, Sigma's lens caps both front and rear work well and fit correctly.
Build
As mentioned, the focus ring has a flaw if squeezed. So don't squeeze it. The mounting ring is metal, not cheap plastic. The rest of the lens is plastic. It doesn't look expensive but it doesn't look bad. It looks better than Sony's cheap alpha lenses. Focus is internal, nothing spins or moves externally when using the lens. There is some noise in the lens even in manual focus. I suspect this is the focus motor maintaining the position of the focus group and/or aperture. I also suspect this is done electromagnetically (which would explain the nature of the noise. It sounds like a tiny speaker picking up interference and a speaker is controlled by magnets, so….)
Verdict
This is pretty simple. Do you need a sharp lens for NEX-7, but, you can't afford the Sony E 24mm ƒ/1.8 ZA? Buy this. Do you really like the field of view this lens offers? Buy this. The field of view of the 24 vs. the 30 is significantly different. Remember, the relationship between focal length and field of view is non-linear. The shorter the focal length, the bigger the angle difference for every millimeter. The difference between 24 and 30 is approximately 10 degrees. Personally, I find the 24 to be far more versatile. The 30 is definitely not "portrait" length so standing slightly closer to the subject or cropping gives the same angle of view. The 24 can be safely used at ƒ/2.2 for great sharpness to compensate in DOF terms. The 30 does not offer much that the 24 does not offer and as such is not a replacement, for any price comparison. If you can swing the $1100 USD for the 24mm, buy that. The Sigma 30mm is not smaller enough to consider it as a compact alternative.
At the end of the day, the Sigma 30mm F2.8 EX DN is a great lens, irrespective of price. It is the third best lens for the NEX system right now, in my opinion (the best is the 24mm, the second is the 50mm ƒ/1.8 OSS). It is the sharpest, in controlled lighting situations, if only by a tiny margin. It is lighter and smaller than the other two. It is one of only three E mount lenses that won't make your NEX-7 sensor look like a snob. But, be warned, it's a Sigma lens and I got a good copy. My copy is pretty much perfectly centered. I won the lottery. Almost every Sigma product is riddled with quality control problems. If you buy this lens and it isn't as good as I say, return it and get a different copy. Do this up to 5 times. Yes, seriously, 5 times. Sorry, that's the price to pay to get the price you pay for this lens.
Eventually I will post some images I made with this lens. You can find many examples around the web for doing your own pixel peeping comparisons, but, suffice it to say, if you have not had 5000+ exposures experience with the E 24mm, it will be very hard to make a reasonable comparison.
But hey, it's $200 USD. (Well, $220 lately) so you won't be taking much of a risk in buying this lens.
Recommendation
Buy this lens if you don't have or plan to buy the E 24mm ƒ/1.8 ZA.