When I was a poor volunteer living in Cleveland, my housemates and I sometimes came to Marc's to supplement our normal Dave's shopping. Scratch that. My housemate Katherine and I sometimes came to Marc's. Actually, she came quite often, and I sometimes accompanied her.
The first thing you need to know about Marc's--and it applies to all the stores I've been to, three in all I think--is that it's dirty and disorganized. I'm sure the low prices come partially from the lack of staff. Or that the staff aren't paid very well. So if you like shopping in clean stores, you should probably avoid Marc's. But it's not depressingly dirty, a la Aldi; Marc's is just disorganized. It perpetually looks like they just had a huge sale and haven't had a chance to clean up yet.
You can get some pretty good deals here, but that's about it.