I stopped wearing my underwired bra on the advice of my wonderful bra fitter. She just said it would be more comfortable to get out of underwire and that because my boobs might grow so quickly the underwire might end up making me uncomfortable, even with proper fitting.
I was 5 weeks pregnant when I went to be fitted the first time, and my boobs were really engorged and painful and the bra I was wearing, (although supportive) was no longer fitting properly. Taking it off and putting on the bra she suggested felt so good I didn't look back.
I'm not wearing maternity bras though, I'm wearing really supportive sports bras and although they're not what I would wear if not pregnant, they give good support and a decent silhouette.
That said, my boobs have now gone back to a DD cup and aren't much sore any more, so I imagine if I'd stated with underwires it would have been grand. But that day the soft, supportive, wide-strapped sports bra was like a gift from the boob gods.
I think the main thing is going to a shop where you will be fitted by someone genuinely knowledgeable about breasts and bras who can give you good advice and show you different options. I would avoid M&S like the plague as I've always found their bra fitters pretty crap. Also I would tend to steer clear of maternity shops - I used to work in one and the training I got for fitting bras was cursory at best.