Yeah, I like to use SP to get all the random side quests knocked out, when your online friends are a bit more to the point lol. I find that the experience & challenge curve are really well done based on # of players etc. Though I must say, I'm leveling just a hair faster in single, and 3-4 player was more of a challenge and experience was only slightly more diminishing returns. I like the added challenge for 4P though.

yeah that's why i'll probably be online i like challenge of having 4 ppl

Well, I slightly retract my previous statement. We were playing 4P, but one was level like 7, so it was really 3P and a low lvl that could revive at best.

is there any grinding at all?

As in like, ass grinding? Yes
As in level grinding? Yes, but it's not really necessary.

The levels flow pretty good since there are challenges (shoot X bullets, make x dollars, kill X enemies, kill X enemies with this weapon type) that you get xp from, and each challenge has usually 1-2 ranks, ex: Shoot 1000 bullets = 1000 xp, shoot 5000 bullets = 2500 xp, shoot 100000 bullets = 20000 xp, so stuff like that makes it very smooth and keeps you right on par. Also, doing quests grants xp usually like 1/4-1/2 a lvl, which makes it go pretty fast, since the game is always fast paced.

i tried renting it but i guess 5 minutes after they put it on the shelves all 10 were gone