Here are the details and my opinions. I will start with the cons.
The biggest con is the sounds. I don't know what IL2 sounded like, because I have never owned it, but PF is made by the same people. The planes sounds, gunfire, etc. are pretty generic. If I could ask the Maddox team to go back to the drawing board on one thing, it would get your tape recorders back out, go to the Reno Air Races and record some real P&W's and Merlin/Allison engines.

The pros:
If you are running FS9 well, then you should have no problem with PF. I run FS9 an average 20-30 FPS with my sliders maxed, except for clouds. PF looks and runs great on my machine. You can see my system specs below.
The scenery is beautiful. The water is awesome, the islands look like paradise, the clouds nice a puffy hardly a frame hit when you fly through them. The South Pacific looks like a real nice place to visit if there wasn't a war going on.

I spent the first 1-2 hours just tweaking the sim to get it looking just right.
The controls and hot keys come only half coded. They let you program the hot keys to be what you want. I like the fact that I can customize my keys from the start so they make sense to me.

The planes all fly very different. Each aircraft has a description about it characteristics in combat, and the coding is pretty close to the truth. For example I tried dogfighting in the P-39, but in real life this was not a very good dog fighter against a Zero. I was slow in the turns, and not able to reach out with my cannon and machine gun. Better as a ground attach aircraft.

Granted I am not a combat pilot of WW2 aircraft, but if I had to guess what it would be like, PF is pretty close.
The difficulty settings are very detailed. All the aircraft will spin and stall realisticly. First combat mission I went on I tried to loop behind the enemy and lost a lot of speed. I ended up stalling and spinning out of control. It was very difficult to recover, but I did. 5 minutes later I was shot down and waiting for S&R.

Each aircraft is very detailed inside and out. Some aircraft come with varying paint schemes, etc. The only thing missing from the interior is the old dingy aircraft smell. My favorites so far are the Corsair, P-40, and Hellcat! The P-40 like the P-39 is a dog in combat but challenging non-the less. Plus you have about 40 planes to be able to fly. At $40 for the game, you get a lot for your money. It comes with campaigns that seem dynamic. You can also create your own battles. I would be interested in recreating some battles, but that is a whole other art in itself.

What else can I say. There is a great forum over at
www.pacific-fighters.com where people are more than happy to help. Not as great of a forum here. I find this to be the friendliest forum in all of flight sim, but all in all a good source of information.
If you have any other questions about PF, let me know. I am not an expert, but might be able to assist.