I used Similac and Enfamil, regular and sensitive. Enfamil seems thicker than Similac, but it made Marin really gassy and fussy and she spit up much more.
Similac sensitive is the best for Marin, she eats more and sleeps much more restfully with less gas and spit up. Enfamil Sensitive isn't terrible, but it clumps really, really bad to the point it would clog her nipples and coat her bottles with thick chunks. She didn't seem to mind though, so that's a personal preference.
There's really no such thing as "bad" formula from what I've read. Formula is so universally regulated that it's basically all up to parents choice and what baby likes. Some have corn syrup instead of lactose and other minor differences like that. Nutritionally, they're all basically the same from brand to brand.
Apparently the difference between name brand and store brand formula is the fineness of the formula granules and how many times it's been sifted.
Don't quote me on any of this, it's all what I've figured out on a very steep learning curve over the past week or so.