Try the demo. You might also want to try the demo for its sequal Arma (Armed Assault). I initially skipped OFP and went with GR1 gold, then RvS and SWAT3/4. OFP was way too complicated when I was getting into tactical FPS but now its one of my favs.
Its realistic like GR1 with mostly 1 shot kills, though you can sometimes survive a shot and be wounded, which affects how you shoot/move. You can heal at medics or medic tents but there are not health packs lying around. There is some time involved, though not much, but its is not instant. Its enough to get you killed if you try to get healed without having cover.
To me, its like GR1 but with larger combined arms forces. Though you can still have solo or small group missions too. Its up to the mission maker. Both OFP and ArmA also have great editors that allow easy missions (just throw down the units you want to fight on the map and go) or complicated missions using scripting (takes time to learn but there are tons of tutorials on the net). I just do easy ones but use a mod that helps to randomize enemy type and placement.
One main thing, at least with OFP, is you only get 1 mid mission save. You can use a cheat function to overwrite the one save though so you can save any time, but you just get 1 not multiples save files. Arma on the easier settings gives you the 1 save but lets you overwrite it without using the cheat function. The controls can also be complicated at first, especially if you are in control of a large squad.
There are a crazy amount of mods that affect gameplay, change the models, new missions/campaigns, countries involved, time period, etc. While the mods can be a bit tricky to get up and running the official forums have tons of info on how to do it. The official forums (check the mod section) and the mod sticky here have some good suggestions.
If you do get OFP, be sure and get the Gold version (AKA GOTY), which includes the 2 exp packs: Red Hammer and Resistance. Many mods require this now. I was able to pick up the GOTY version for $2 off ebay. Xfire shows me past 50hrs. Almost finished the original campaign and have spent tons of time on custom missions and playing in the editor making my own. Would have still been worth the full price to me, which is one reason I picked up ArmA on release knowing it would need some patching, time to mature and time for mods to be made.