Avatar is a solid gaming experience. There is absolutely nothing in this game that keeps it from being a lot of fun. The graphics are spectacular and the story is quite compelling. The Na'vi and RDA campaigns play like completely separate experiences which surprised me. They take place in different sectors of Pandora and each one is distinct. Playing as an Avatar, you tower over the humans and the plant life. As a human you are dwarfed by the plant life, Na'vi, and animals. The RDA campaigns really give you a sense of how hostile the environment is.
I have played through all of the Na'vi campaigns and over half of the RDA campaigns and I feel that the RDA campaigns are definitely harder. One of the nice things about Avatar, though, is that it is not a hard game by any means. You don't die much and you shouldn't have to play any of the segments multiple times to succeed. This may irritate hard core gamers but as a less skilled player, I consider it a plus.
Avatar is a good game but with a little more polish and a few adjustments it could have been steller. While the graphics are solid, there are still a few occasional glitches. In the later campaigns there is a little "drop-in" from time to time and throughout the game AI controlled Na'vi and RDA soldiers will appear from a long distance as featureless moving dolls. Then when you get closer the details will snap on. This is hard to explain but when developers make a game the character models are like fleshed out stick figures with a "skin" added later. It was weird and mildly irritating during the game to see characters moving around in the distance as little white figures and then once you got just a bit closer, "pop," now the details. The only other glitch and one common in many games is that some AI controlled characters would get stuck running in place against some of the scenery and once I saw an arrow get stuck in midair. Also the dragon like creatures the Na'vi fly called Banshees are sometimes seen perching in midair. No big deal but silly nonetheless.
Speaking of AI. Sometimes the enemy AI (wheter Na'vi or RDA depending on which side you play) is dumb as a stick and other times so clever you have a hard time staying alive. I would have preferred the AI to be better thought out. The developers should have either started the AI out as pretty stupid and then gradually smarter as you progressed through the game or make it class specific. For example, some the Na'vi tribes would be smarter or tougher than others with variations within tribes depending on ranks and on the RDA the AI could be assigned based on rank (grunts, commandos, elites, etc.) Either approach would be better than what was actually done.
The controls, weapons, skills, and vehicles are pretty well thought out but a few changes would have made them more fun. Both sides can equip 4 weapons and 4 skills at a time. However, each side has one default that you cannot swap. The RDA has twin pistols with infinite ammo that are almost useless and the Na'vi has a bow. Now the bow is a great weapon that you will probably use regularly throughout the Na'vi campaigns so it isn't really a problem but the pistols are a wasted slot. When you play as the RDA you need all the firepower you can muster. The skills are good but unlike the advertising there isn't 60 weapons and 20 skills. You have a handful of weapons and skills with 4 automatic upgrades for each one as you gain in experience points. The Na'vi have 6 weapons and if you count each upgrade you technically get 24 but they are not really that different just more powerful. The Na'vi have 9 skills and the upgrades bring you up to 36. The RDA have 8 weapons and 7 skills. Each side has a sprinting skill but a dedicated standard sprint would have been nice. The left joystick button isn't used so they could have made that the sprint which would have been nice. The RDA vehicle controls work just fine although the AMP suit is not much help if you get swarmed by Na'vi because they are so much faster. The suit gets destroyed in seconds and if you don't get clear of the blast you're toast. Also, I don't understand why when you run over the Na'vi they aren't killed. Usually there is minor damage if anything. The animal controls are okay but it would have been nice if they could have figured out a way for you to use weapons while riding. Also, it is hard to land the Banshees even when it looks like a spot has room. You keep pressing "Y" but the stupid thing won't land. This is major irritation.
Two final things that I can think of at moment that could have used some improving is the campaign structure towards the end of the game and enemy spawning. The last Na'vi campaign seemed rushed. In the rest of the campaigns the quests were spread out across the maps in such a way that you would have explored most of the map by the time you finished the sector. To complete everything I had to go back after I beat the game and explore the rest of the map. The developers clearly got in a hurry and I am curious to see if the final RDA campaign is structured the same way. A big improvement would have been to completely separate the campaigns. This would have aided the replay factor big time. What I mean here is that you have to play the opening sequence twice if you want to play both the Na'vi and RDA campaigns. You start out as an RDA soldier and after you are transferred to your Avatar body you come to a point where you have to choose sides. Take one route and you stick with the RDA and lose your Avatar, take the other and you turn on the RDA and remain in your Avatar body. It works as a story but it is boring to have to do the same stuff twice in order to get the point where you can take the route you didn't take the first time round.
Now regarding enemy spawning. Sometimes you clear an area and then you have to travel a ways before the enemy respawns. Other times they respawn so quick you have to sprint away to get clear. Also, sometimes the enemy follows you until you kill them and other times enough distance gets you clear. I would have much preferred a less haphazard approach. No respawning until you replay a sector would have made much more sense.
The multiplayer is okay. I am not a big multiplayer fan and I cannot see where this game breaks any new ground in this mode. I haven't tried the conquest mini game yet but I suspect it some kind of RTS game.
In conclusion, all quibbles aside, you can't go wrong with this game. It is interesting and fun at the same time. Buy it sooner or later.