I compare the ingredient list and nutritional info of generics carefully with the brand names.
When my youngest was about 4 months old I switched from brand name to generic formula. We used the Kroger comforts brand (the regular version of it, not the lactose-free that rkhyclak used), we also used the Walgreens version which says 'compare to similac advance', and a Wal-Mart Parents Choice brand.
I think the store brands are fine. My baby didn't have any issues switching between different brands. Although there are so many varieties within each store brand that you have to really make sure you're comparing apples with apples & selecting the correct one for your baby. (I did find it hard to compare all the nutritional info in the store with a baby and toddler in tow!!!)