I started playing with .ini files and I was hooked from then on. Now, whenever I play a console game and there is something about how the game plays that kind of rubs me the wrong way, all I can think about is how can I tweak it to make it play better, then realizing that I can't tweak anything, and then I get sad. Also, mods. Mods can be pretty cool.
It really depends on the game. I bought Far Cry 2 and am going to buy Far Cry 3 solely for the single player experience. Same for the Bioshock franchise. But there are games where the single player is just a glorified tutorial getting you ready for the multiplayer, like Red Orchestra 2.
Edit: I forgot to answer your question. I would and do pay the 60$ price for amazing single player experiences without multiplayer, so I don't know why I wouldn't pay 60$ for an amazing multiplayer experience. (Which I guess I have with things like Wow and LotRO. Not FPS, but the same principle applies.)
The story is pretty forgettable, but I really enjoyed the fighting system once I figured out what I could about it. The way your characters upgrade is insanely complicated but interesting. One thing I learned is never grind out levels. If you do, it will make the game impossibly hard. That being said, don't be afraid of fighting all the monsters you see or doing side quests. Just don't go out of your way to grind. I've put a good 50 hours or so into this game and enjoyed it completely. If you do get it, here's a few tweaks to make it play a bit better.
EDIT: also, don't be afraid to explore the world before doing main story quests, it's the only way to find side quests and cities
EDIT2: and remember to fight as many enemies at once as you can, and try to keep your battle rank as low as possible. It's madness I know, but it's just the way the game works.