I was exclusively breastfeeding my DS. At 16 weeks he was still up every 2 hours in the night for feed and same during the day. I was a zombie.
I bought Aptimal hungry baby formula and give him a bottle of this at bedtime, but breastfeed the rest of the night and day. It did make a difference and although he would still wake for a feed it was once or twice a night. I'm not sure if normal formula would have had the same affect, but the aptimal is meant to be closest to breastmilk, so didn't think substituting breastmilk for something simmilar would help.
He is now 8 months old and we still give hungry baby formula at bedtime. We did not every have any issue with constipation etc, but probably because it was only one bottle with breast milk rest of time.
I don;t think you "have" to go on to hungey baby formula to delay weaning as seems to be suggested, I think it is just an option if you have a hungry baby who seems to need more, but too soon to wean.
My HV were quite sensible about weaning ages (I know this is a hot topic at the moment) and rather than the strict 6 months, they said anytime from 24 weeks. I think I started DS on baby rice at 23 weeks, and then puree, but very quickly went on to BLW and have been doing that since. DS is 8 months and over 22lbs, so seems to be going ok. I didnt ever feel that he needed solids before this age.