In order to understand what DCS World is, you must first understand what it isn't: DCS is not a fighter combat game. If that's what you're looking for, go download War Thunder; you'll have hours of fun with it. DCS is a high fidelity simulation geared towards pilots, sim-heads and plane geeks -- of which I am all three, therefore I absolutely adore it. If you fall into that same category, go ahead and push the download button then come back to finish reading. If, however, you're an average gamer looking to jump into a plane and start blowing away the bad guys, it's very likely that you'll be sorely disappointed with DCS.
The focus in DCS is realism, literally down to every switch, knob and light bulb. The people who love this simulation crave detail, and it provides that in spades. But you must love to fly. I don't mean you glance at planes occasionally and think, "That's cool." I mean you have to be the sort of guy who spent hours and days and months as a little boy thumbing through books about planes, watching television shows about them, and generally absorbing every detail you could. You skipped meals and let your rent run late as a young adult so you could afford your next lesson in a beat-up old 152 at the local asphalt strip. You have strong opinions about the viability of the F-22 vis-a-vis the F-15 on a cost/return ratio. In other words, you eat, sleep and breathe aircraft.
DCS is a plane hobbyist's dream, and it is deliciously difficult, providing an enormous feeling of accomplishment for every task: cold starts, taxiing, take offs, radio navigation, and landing are just a few examples. The combat is simply the icing on the cake; the real object here is learning to fly the various aircraft in all their hyper-detailed glory: cussing when you burn out the starter on the P-51 because you forgot to turn the fuel booster pump on; reveling in a successful crosswind landing on a short strip at dusk; setting up a clean Maverick attack and executing it perfectly then rolling off into a valley; or finding an airfield at the last possible moment and bringing a damaged bird in on fumes and prayers.
I suppose you could use the arcade mode settings, but I can't for the life of me imagine why you would choose to do so. It would be missing the entire reason that DCS exists. That's like buying a liter bike and putting training wheels on it. There is no dynamic campaign, there are damned few missions, and there's no progression model, (and thank God for that). It's a simulator, not a game. The planes are the stars; the handful of pre-made missions are just there for when don't feel like creating your own. Sometimes, just flying a radnav loop above a thick overcast can be relaxing.
As for the individual aircraft modules, I own a majority of them. They are all uniformly excellent, though you should stick to the aircraft which use the pro flight model and fully interactive cockpits. Several of them, such as the F-15 and Su-27, are just ports from Lock-On; avoid those. Of all the modules I own, the F-86 is the one I keep coming back to time and again. It has a special charm, being at the leading edge in the early days of jet-vs-jet combat. The sound is stunning (it sounds exactly like a real jet turbine; crank up the surround sound), and the plane is easy to learn. Note that when I say "easy," I've thousands of sim hours and hundreds of civ-av hours. I adapted pretty quickly, but if you're new to realistic simulations, it will probably take you several hours just to figure out start-up, taxi and take-off. Once learned, however, the plane rewards you quickly, with a feeling of speed unparalleled in simulations to date. A dogfight versus a MiG-15 will give you an appreciation for the high speeds and energy envelopes involved, the enormous scope of the battle, and the difficulty of hitting a small, fast target with nothing but machine guns. It's less a knife fight and more like a ballet -- if ballets were performed on salt flats in Formula One race cars. Note that I recommend you start out by learning the art of jet-powered gun passes against an unarmed transport. Practice that for several hours before moving up to targets that shoot back. Otherwise, you'll be watching that MiG from a parachute.
A couple other modules which stand out for their excellence are the MiG-21 and the UH-1 Huey. I have reviewed the latter separately; search my review library. They're all good, of course. The best part is that you don't have to buy a single plane to get started: the Su-25T and the TF-51D are included in the free download. All you War Thunder mouse heroes, try the TF-51 and prepare to be humbled. You just THINK you know how to fly.
As with any high fidelity simulation, I strongly recommend HOTAS equipment with rudder pedals, as well as TrackIR. You'll have to spend some time setting up your controllers and tweaking your curves. Accept this right now, or you'll get frustrated. This is a program for adults, not impatient children. Also, DCS provides excellent support for triple-monitor systems like Nvidia Surround, you're golden if you're a superwide or ultrawide resolution guy.
Hey, it's free; try it out. Just set your expectations for what you're getting into. If you're looking to be shooting down the Hun five minutes after the download completes, you'll almost certainly be wasting your time here. But if you had internalized "lifty-thrusty-draggy" by the age of 12 and are still willing to engage in a lengthy debate about the relative merits of the XB-70 Valkyrie program, you'll adore DCS. It was made just for you. Welcome home.