I just installed Project CARS today (13 May).  And, of course, my first impression is WoW!!

After toying with it for a couple of hours, I wanted to offer these observations...

• The UI is not always so intuitive.  I found myself needing to employ more trial and error attempts than I would have imagined to figure out how to get around in it.  I realize a significant part of my frustration stems from the fact that I got so used to the Forza apps before PCars came out.  I'm sure this will get easier with time.
• In a similar vein, the controls are significantly different than what I got used to with Forza - most notably, the manual shifting.  In Forza, it's the right joystick, with forward for upshift, backward for downshift (at least that's the way I have it set), allowing me to shift with my thumb while keeping my right forefinger on the right trigger (for acceleration).  In PCars, upshift is the right button, downshift is the left.  That forces me to move my finger from trigger to button in either case - accelerating/upshifting, or braking/downshifting.  I can't imagine a real-world driver taking his foot off the brake pedal coming into a turn just so he could downshift through the gears.  But then, said driver isn't using a stock game controller is s/he?  I'll admit that if I had gotten used to the PCars way first, then switched to Foza, my complaints might be directed at Forza.  But still it seems like PCars could have separated those functions and not made us use the same finger(s) for simultaneous functions.
• Re: actual driving (i.e. car physics), I can already tell there's going to be a STEEP learning curve with PCars.  I never finished higher than 15th (out of 20), even when I started in 2nd place.  The red/amber/green (and all shades in between) indicators on the driving line are strict.  Again, in Forza (coz that's what I know), you can usually push the envelop, sometimes significantly, and finesse your way through a turn even when the driving line is in the red.  But in PCars, if you see even a hint of yellow, you will probably go wide at the very least, and if you drive on a red line, expect to be kissing a wall in the next few seconds.

My overall impression is, of course... OMG! This is gonna be great!  I'm convinced already that a wheel/pedals configuration would make it much better, but I don't have the space (or the funds) for that right now.

I've read posts by some members who talked about the steep learning curve associated with iRacing, and it seems that PCars is much the same.  I guess the closer a racing sim - any racing sim - gets to really duplicating the real-world driving experience, the more that will be the case.  I mean, I wouldn't expect to go out to Circuit of the Americas, jump in a high-end race car, and be able to set an impressive lap time on my first go, so I shouldn't expect to in PCars either.

Please share your impressions as well.