Well, heres my feedback for the demo.
Basic OpinionMy opinion, on the demo, is that the game is at a great stage in it's development. The demo itself is surprisingly good, and even with just one engine type, the addictiveness of the demo itself is extremely high. There are a few bugs here and there; most have been mentioned in the bugs thread. Updates will most likely fix them. I've spent much of my time in the sandbox mode, as scenarios aren't really my thing. I usually play the "free" mode in a game (meaning no limit) or the "campaign' mode, and in the full game that will be running my own car company.
SoundsThe sounds in the game are okay, but I'm not going say good or perfect for a couple of reasons. I own a 2011 Scion tC, and some of you may know that by looking at my profile already. It has 180 BHP, and it redlines pretty much at 6100 RPM-ish in sport-tronic shift mode. Now, heres my complaint. I recreated the (almost) exact same engine in the engine designer demo. It had 180 BHP, and it redlined at 6100 RPM. The result? An engine that made my Scion sound alien. It was honestly way different then my real life engine, sounding like a bunch of angry bee's being shaken in a can (aka a rotary). It is just a game, but Forza 4 had a shockingly similar engine note.
The exhaust note is a somehow entirely different story. When I moved the camera at the exhaust, I could imagine my brother revving the engine, and me listening to the throaty sport exhauast on my real Scion. So overall, the engine note (in my opinion) needs some tweaking. Well, not the engine itself, but the sound the engine makes when combined with certain parts. Now to other misc. noises. The sound of the garage/working room is pretty realistic. It reminds me of the automotive class that I took in High School basically; the sound of fans and the clanks of tools. (That's a good thing!

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ControlsThe controls of the demo are basic, yet extremely hard to master. You start by placing an engine block, determining it's size etc., but then you start finding yourself tweaking various components of the engine and the parts attatched. If I'm not satisfied with my 5600 Redline engine because it's not producing optimal results, then I can easily increase it to 6600. Easy as pie. But, the hard part is finding the perfect setup. Do you want maximum economy ratings without a loss of performance? Then spend the whole afternoon tweaking and changing to find what you want.
The camera controls are pretty basic. Drag and click, zoom and scroll. But again, the game is complicated under it's shell. You can use the camera to zoom into the exhaust to hear a clearer sound, or scroll around the pistons and watch the sparks and flames. It's pretty awesome.
Overall For a demo, it's extremely addictive. You can change seemingly everything about your engine. The only thought I'm left with is: "Can the Car Designer live up to the greatness of this single demo?"