I was NEVER a fan of flats (just what I had hear of them...I never had a set on my bass) until a few months ago. I decided to pick up some Chromes and put it on my fretless Jazz. They really are nice and they dont sound all thud to me at all. Just for grins, I put on a free set of rounds I got recently on it and I am going to take them right back off. The flats work so well on it. I may try a set on my 5er too. I too find that its not good to judge them until you are in a mix with a band. It does help me get below the guitars a bit without competing with the zingy overtones.
Keep in mind that I still have rounds on my main bass because I am in a cover band and need that sound more in the songs we do, but one day I might go all flat.