Its all a personal sound choice and what style of music you're playing. Blues, Reggae, RnB, Soul, basically anything with an old school tinge to it, they'd sound good. I prefer a deader sounding string but thats just me.
They can work in rock reasonably well because they can give a bigger bottom end and can be useful in other situations where a bottom end needs filling out like Musicals - but EQ'ing can achieve this too.
But be warned, the B string on flats arent the best - the stage II preamp would help nicely here.
As for brands, I use both Thomastik-Infeld Jazz Flats (.43 - .136) on the Warwick and LaBella Deep Talkin Flats (.49-.109) on my P Bass. If you're used to the normal 45-130 odd, go for that style gauge. The heavier the gauge, the harder it is to play, especially seeing that most flats are stiff as anything. The TI's are a weird feeling strings, its a similar stiffness to rounds on your plucking hand and then like elastic bands on your fretting hand - their G string is a bit twangy though.. Great strings none the less, Ive had a set on the Fortress for around 2 years now.
Ive heard good things about Rotosound Flats, I personally wouldnt touch Fender, GHS or D'addario Flats.
Bare in mind that you may have to file your bridge and nut to fit the B string in. Thats the only hassle.