There's no need to feel guilty, but there's also no need to completely stop breastfeeding if you enjoy it. My lo is 11 weeks old, and we're mix feeding fairly successfully. I breastfed exclusively up until 3 weeks old, but it got to a point where I was dehydrated and frazzled, and he was either feeding, or screaming for milk I didn't feel I had, probably because I had no time to cook proper food or look after myself, it was a vicious circle. The pressure was stupid and unnecessary.
He also wasn't putting on any weight, or shifting his jaundice. So I started to give him the odd supplementary feed, and he finally lost the jaundice, and started to put on some weight. We are still mix feeding, although it's about 75% formula now, it means there's absolutely no pressure to breastfeed, so it's nice just to be able to feed him whenever I want during the day/night. He's still getting my antibodies and plenty of bonding time, and I get more freedom when I need it. The cooking/shopping/cleaning don't do themselves yet unfortunately!
I'll be honest- the cost, the extra washing up, the sterilising, and the planning ahead is all a pain in the arse, especially after the utter convenience of breastfeeding. And lugging about all the feeding paraphrenalia just so I can go out for an hour is a real pain, BUT then again I can't really breastfeed in public anyway because I can only feed him in the football hold with him supported with a million pillows. Not easily accomplished in your average mother and baby room, never mind in a cafe.
I don't regret mix feeding now (after a week or so of feeling utterly guilty initially) as it has made me enjoy my time with my baby so much more now the pressure is off and neither of us are in tears, and can enjoy breastfeeding without the worry of "am I producing enough?" "is he gaining enough weight?"
I think we have the best of both worlds. But remember, the bottom line is that no-one in real life will comment on what you choose, so do what suits you and your baby, don't feel guilty just because of MN.