[Dysko]

Recently I came across a copy of Thirdwire's Strike Fighters Gold. I quickly bought and tried it.
It's a very nice nice combat flight simulator. The graphics is not top-notch, but is good anyway. The flight model is realistic, but don't expect a Falcon 4-esque sim. Avionics are simplified, but weapons are still differents in the way they work: you won't need to push 300 buttons to take-off, but you can't expect an AIM-9 Sidewinder and an AIM-7 Sparrow to behave in the same way... Also, this sim focuses on the first times of missile dogfight. This means that missiles (especially if you choose early models, like the AIM-4 Falcon or the AIM-9B) will be very unreliable. And this means you'll have to fly your heavy F-4 Phantom in a dogfight against the light and very maneuvrable MiG-17 most of the times...
The planes featured in Strike Fighters Gold are the A-4 Skyhawk, the F-100 Super Sabre, the F-4 Phantom and the F-104 Starfighter (if your copy is not patched to the last version, the F-104 is available from Thirdwire's website).
The campaign is set in the fictional countries of Dhimar (allied with USA) and Paran (allied with USSR) in the Middle East, and it's fully dynamic (though it's very simplified, don't expect Falcon 4's campaign engine). You can choose to fly as a USAF, US Navy or mercenary pilot for Dhimar (Strike Commander or Area 88 anyone? ).

Then I discovered that Thirdwire developed other sims based on the same engine: Wings over Vietnam, Wings over Europe (a fictional NATO vs Warszaw Pact engagment in Germany in the '60s and '70s) and First Eagles (a WWI sim). SFG, WoV and WoE are modular: you can merge them together, and get a "bigger" sim with all the airplanes and theatres. Also, when a new sim comes out (Wings of Israel will be the next installment of the series, and will come out soon!), the older ones are patched up to reach the last sim's level (i.e.: when WoV came out, SFG was upgraded to support carrier operations. When WoE came out, SFG and WoV were upgraded to support '70s avionics).
After SFG, I got also WoV and WoE, and they're very good!

Another big advantage of this series is that it's very open to modding: you can find lots of add-ons planes (from WWI and WWII warbirds to F-14s) and add-on theatres.

If you want to get more infos on the sims, visit Thirdwire's website.
If you want to take a look at the add-ons available, CombatAce is the best place to look for them (it has even a sci-fi section, where they've been talking about BtRL and WCS).

[Polpolion]

Is it messed up like Ace Combat 6 where you can mount 80 air-to-air missiles on an F-16?

[Mobius]

Wrong...you can mount air-to-everywhere missiles in Ace Combat and destroy tanks with Sidewinders...

[Dysko]

Is it messed up like Ace Combat 6 where you can mount 80 air-to-air missiles on an F-16?

Had it been like Ace Combat, I would have nuked the place where it was sold I think that the Ace Combat series is racing with the Iron Eagle series for getting the "Worst airplane-related crap" award

Loadouts are realistic (i.e.: first versions of the Phantom don't have an integrated gun and have to carry a gun pod, Starfighters carry no more than 4 air-to-air missiles and 2 droptanks in air-to-air missions etc.), but I think that changing an .ini file for each airplane could enable Ace Combat-esque loadouts. Fortunately, I haven't seen yet airplanes modified in that way.

[Unknown Target]

Thirdwire's Strike Fighters: Project 1 is actually pretty known in the flight simming community, due to it's pretty good realism, and it's extensive moddability. It had a rocky start but I hear it really turned around is just awesome now, especially with the SCP-esque upgrades it's gotten from the modder community.

[IceFire]

Looks pretty good...it'd be awefully hard to break IL-2's grip on my simming time but I'll have a look at it!