I also write the melody first, lyrics second. I've actually done both ways and found the flow of it all was better with melody first. Although, that's probably why I'm an amateur and not a professional! One of my favorite parts of that method has been described above. I enjoy having the music going and just "freestyling" lyrics and usually, something does pop out that I want to keep in there.
I'm hoping not to offend here but my opinion is if you're trying to write a hit song, the melody is more important. Here's my thoughts on why.
How many times do you hear a new song on the radio, mtv, etc... and have the words to read along to? For me, I never do for a new song. When I'm driving, working, at a bar or anywhere and I hear a new song, it's the melody that hooks me. Not to say that while I'm listening I don't try to understand the lyrics and try to figure them out. Later on in the day, or next day, I'll get to humming or singing in my head that song I heard that really stuck out. I don't usually remember the words, and sometimes the melody isn't exactly right, but it's the melody that has me humming. I, as most of you, have read hundreds upon hundreds of lyrics on sites like this that, to me anyways, are better than a lot of the hit songs I've heard. But even the greatest lyrics, without a great melody aren't going to get too far.
In the end, the best songs out there are the ones that have the whole package, great lyrics, great melody, great voice, etc... Top to bottom it's the full package. But, there will be a hit song that has so-so lyrics and great melody, before one that has great lyrics but a poor melody. Consider Smoke on the Water. I love that song and have for over 20 years. Funny thing is, I just learned what it was about a month ago on a VH1 Classic show. I never had any idea what it was about but it always flowed together great, sounded good, and was fun to sing and play on guitar. Not to say I hadn't enjoyed the lyrics just never understood them. I guess that brings up a new question. What defines "good/great" lyrics? Think I'll have to start a new topic for that one!