I haven't used their formula, but we LOVED their solid foods!! they just had a segment recently on the Today show on the amount of pesticides in baby food....16 alone in gerber's! and that was the brand with the least. their expert recommended Earth's Best foods.
in regard to the BMs, remember it is common for infant's bowel habits to change upon aging milestones.....usually I think it's 4-6 months (depending on when on starts introducing solids, the child starts sitting, etc.) however, if you're switching to formula that could be a red flag to watch for, certainly. Formula is much more constipating than breastmilk, and quite honestly I don't think that particular side effect will change whether you're using similac, earth's best, or the Kroger Brand (which I believe is actually manufactured by similac).... but you have to do what you're comfortable with.
fyi, I recently had a nutritionist tell me that they are linking ADD and ADHD to soy based formulas and milk received in the first two years of life....course like autism, those findings change all the time.
I would check with your pediatrician's office for your best advice on your concerns.
Congratulations!! Good luck :)