I was a beta tester for MS Flight. I have to confess not a very good one as I quickly became bored with it and stopped using it. By quickly, I mean within a day or two.
It is very pretty to look at and very smooth running but the whole thing appears to have no depth to it. As many have said here before it is most definitely a game, with a very short learning curve. Great if you want to get some instant gratification and look at the nice scenery, but if it is some serious flying that you want, then it is not for you.
Frame rates are great, even on a mediocre PC, but there seems to be very little to do except circuits in the limited space available. We may see new scenery areas for sale as add-ons in the future, but what if you want to fly long haul? will you have to spend a small fortune buying your departure point, you landing point and everything in between? will you even be able to fly between two points that are in scenery sets that are unconnected? Too many questions and no answers, but on the face of it Flight seems restrictive, (perhaps in its ability to support third party additions as well as its airspace), and frankly dull.
Personally I think MS would have been better rebuilding FSX to take full advantage of modern chips and graphics cards spread the load and improve on what they have already got. It worries me that Flight will not be as popular as the FS series for the reasons already posted here and will be canned as a result, with nothing to replace it.
FSX looks like it will have to satisfy those who take this hobby seriously for some years to come. In my opinion MS have made several error of judgement with Flight, and we may all pay the price of their attempt to make more money out of their product.