I'll only list the negatives, compare it to DCS & explain the Module System IL2 uses in order to make it easier for people to choose

- Very unfriendly towards beginners
- Shallow learning curve (long progression)
- Modules are rather expensive
- Flightstick & TrackIR or VR needed to be an effective pilot
- Communication menu between you & AI in your flight is very lackluster (basic commands only)


This Game is more of a Platform than a Game
IL2-Sturmovik is centered all around WWII. The "game's" DLCs you see in Steam are not really DLCs. Theyre not even expansion packs. Theyre Modules. What does that mean? The Game (or Platform) as a whole (with all modules), is called "IL2: Great Battles". The Module "IL2: Battle of Stalingrad" was only the first and also the main Module.
Thats why Steam classifies BoS as the "main game" & the other modules as DLCs
Every Module has the same amount & quality of content: A new Map, new campaign, quick missions & new planes.
Only difference is the theatre of war & the planes that come with it. Every Module centers around a major air battle in the war. You have Moscow, Stalingrad, Kuban bridgehead & Operation Bodenplatte for now. Except for Tank Crew which is centered around Prokhorovka and Tanks.
This is also the reason why the Modules can cost as much as BoS itself


TL:DR:
You want to get deep & sweat the hell out of one singular Plane with all its systems & flight characteristics?
Get DCS!
Want an immersive WWII simulator?
Are you fine with planes which only offer about 80% of the realism DCS Planes do?
Get IL2!

Long Version:
DCS only offers 8 WWII Planes: Mosquito, P-47D, I16, Spitfire LF Mk IX, Bf-109 K4, Fw-190 A8 & D9 & P-51D.
IL2 Great Battles has 8 Planes + 2 "collector" Planes per Module.

The WWII content of DCS exists but it has to be bought extra first. All the standard built in Maps & ground assets are purely modern. Turning DCS into an complete immersive WWII Flight Simulator like IL2, can be very expensive since you need a Plane, a Map and the WWII Assets DLC which both cost the full price.

Yes, but actually no

You see, the only differences I noticed are: IL2s Planes simply lacks a clickable cockpit & the systems are not as deep modelled & programmed. In terms of Flight Model, no one is superior. DCS does thing X while IL2 does thing Y better.

My personal opinion on the whole thing, can be easily answered with a dead meme: Why not both?