Ok, well....
I've had it for two days.
To be fair, I'll probably keep it purely because you can fly Spits, Mosquitos and so forth from the UK in a vaguely historic manner. This earns a lot of points. And the campaign engine is good. And I've had no real problems running the thing on a XP1800/512DDR/GF4MX system - despite initial fears it performs well, with only occasional stuttering.
It is not, however, a patch on IL2 in terms of simulation. Even on the most accurate settings, it just doesn't feel half the product. The flight models are oversimplified, and it's not as slick graphically (sometimes IL2 is just jaw droppingly good) or audibly. With the Forgotten Battles add-on coming up I think IL2 will remain top o'the heap for a long time to come.
Essentially if I want to go fly a WWII aircraft and feel like I was really there, it's IL2. Technically and atmospherically, it's peerless. If I just want a quick blast against some Ju88s over Tangmere in a Spit, or a fun campaign over France, I'll load up CFS3.
In addition, I'm sure the inevitably huge array of user add-ons for CFS3 will keep it interesting for a long time. It's probably worth the investment if your system can handle it.