they're basically as you suspect, a slightly relaxed road bike, the dissadvantage of them is, if you've got heavy rain or riding straight into a wind you can't duck out of it like you can on a bike with drop bars, you've also got one hand position, with a drop bar you have at least three (on the levers, on the drops and on the tops) if not more making it more comfortable to be on the bike for a longer length of time (you move about on the bike less than a mountain bike)
personally, I went with a drop bar bike and wouldn't change it for my road use unless i needed to carry lots of gear / kids but that won't be for some time yet for me and not the kind of bike you're after
basically, i'd say go for the drops, you can always fit flat bars if you don't get on with them. if you want to try the difference find a shop that sells two bikes almost the same, one with drops and one with flats - the scott sportster comes in either version, the difference being the bars.