RealFlight G3 is a good "basic" sim. It has nice graphics, a large amount of flying fields and aircraft. Plus it has a large amount of User created aircraft and flying fields. The free upgrade for G3 (G3.5) added things like night flying, streamers and reactive 3D objects. It did little to nothing to upgrade environmental modeling or aeronautical physics over what existed in G3.

G4 added water to the mix along with a "different way" of doing things - meaning that the physics model was changed. Better ground handling was added along with "springing" gear for some of the planes. The ground handling was not a huge improvement over what they had before and the water physics were very "cartoonish". I have yet to see a float plane react to water the way they do in G4. G4 required more computing power, especially in the video card area. Unfortunately, RealFlight wasn't very good at informing their customers of the true computer needs to get best performance from the software. Their minimum requirements meant that you would lose a great deal of graphical features.

G4.5 (a free upgrade for the owners of G4) added the ability to change the "difficulty" settings which was a good attempt at changing the aeronautical physics settings. By choosing the most difficult settings you could now get close to having some real life flight experiences. G4.5 was also infamous for it's problems. In the first six months after it's release it received no less then 15 upgrades in an attempt to correct bugs.

G5 as you're aware has added combat features and little else. There was no attempt at fixing any of the long standing physics issues, instead, as usual the company chose to add more "gaming" features and "eye candy" to attract the younger/newer/first time flyers instead of making a better product. Not to mention the fact that now you need a good quality "gaming" computer to get "basic" graphics and still have the ability to fly. Plus issues like "hitching" (that first presented itself in G4) have not been corrected. (Hitching is when the screen freezes while the program loads - in this case it seems to be caused by the software loading sound files. Most users in an attempt to correct the problem simply crash the first plane they fly, deliberately.) This has been an issue since the release of G4 in Nov of 2007 and KnifeEdge is just now asking customers for help to resolve the issue.

The really sad issue with RealFlight is it's strong armed attempts at silencing any and all criticism of their product. I have given my honest opinion here, yet I've been banned from websites owned by the president of KnifeEdge Software (the software developer for RealFlight) for saying the same thing ... It seems that Hobbico, GreatPlanes, RealFlight and KnifeEdge Software only want to hear praise for their products. That might be why G5 isn't much of an improvement over G3 - except that it has more games to play.

By the way, if you're buying a used copy of RealFlight make sure that the previous owner has un-registered it. You won't be able to register the software unless it's un-registered by the previous owner.