I've slowly been playing the ArmA campaign since the game was released. I got into OFP late (2006 for $2) so was looking forward to an updated game from BIS. While its release was buggy and had performance issues, patches have improved the game a lot as well as more mods have now been released and matured. I finished up the campaign using beta 1.12 and for the most part enjoyed it even if it fell short of eclipsing OFP.
I agree with others that ArmA is more like OFP 1.5: a graphic overhaul, some tweaks and the main change - Join In Progress MP games. I never played MP OFP since it took to much time to set in a lobby waiting for players and I don't feel like scheduling a time to play it. I've only played a bit of ArmA MP, mostly co-op and while I enjoyed it, MP is just no longer my thing. The campaign is weaker than OFPs but I still enjoyed playing it. I liked how there were side missions one could choose to do that supposedly affect the main mission (Not sure of how much the win/loss of these missions affects the main one though).
I liked the beginning and last half of the campaign but there were many missions early on that were too unrealistic (Single man/Rambo VS the whole enemy army). I'm not sure if the patches updated these aspects of the mission but I did notice AI improvements (Behavior, Targeting, Pathfinding, etc.) and fewer bugs as I played some more with each patch. The main mission bugs I dealt with were missions not ending after the objectives were or appeared to be completed so I just used the 'endmission' cheat to see my mission results and go on to the next one.
The game is still clunky like OFP but they are both still one of my favorites. I do like the double tap controls (Hold key down to lean and release to stand or double tap same key to hold lean and retap to stand, so your fingers can go back to other keys while still leaning, etc.) While Ghost Recon (1 not GRAW) and Raven Shield comes close to it and are better for CQB, OFP/ArmA win out for the scale of maps, battles and editor/modding. Its easy to pick the island map one wants to use and then drop infantry, tanks, jeeps, choppers or jets and have a small or big battles. If one wants one can learn scripting or use some of the helpful mods (DAC, Real Time Editor, Scripted Objective System and Kronzky's Urban Patrol Script) to make even more complicated missions. Or just download the user made ones if you don't want to make them yourself.
Here are some of the mods I've been using:
Guide to realism mods - Best guide to what mods to use I've come across. I used a mixture of the following mods during my campaign playthrough based on its suggestions: PROPER mods; 6th Sense mods; True Mods; & Fixed Range Nightvision. I still need to try out some of the others.
I used the in game difficulty and had my Friendly AI at .75 and Enemy AI at .40, which sets skill and precision to the same number. One can also edit the user file to have different skill and precision settings:
I've also started messing with parts of the SLX Mod in the editor. So far the pbos I've messed with were FindCover, Gl3 (AI improvements), Wounds, NoTalk and Clouds. Other good mods from what I read are XAM and ECS.
I've been thinking about making some Mad Max/Fallout/Wasterland inspired missions using these Resistance and New Order units.
Some single missions to try: Evolution & Quarantine & Dynamic War. Lots of others out there as well as co-op missions to play even alone with bots.
Some campaigns to try: The Recluse, Gloria, The Cause, Fedain & PMC First Fight. A quick search of the main forums for campaign under user missions shows more have been released since I last looked around. Still have to try these out before looking up new ones though.
While I've grown to like ArmA more than OFP (I still play both though), I'm not sure of its general lifespan. ArmA and OFP still both have active modding communities but with ArmA2 I'm not sure if people will stay with ArmA or just have ArmA2 and OFP groups. Probably depends on ArmA2's campaign, how quick its modding tools are released (ArmA was slow) and PC requirements.