I love flats too. They are mellower, but also seem to have a much more even EQ response with fat round bass and mids where rounds are kind of peaky mid/treble heavy. They are also just nice to feel, I feel like they make you chill out when playing.
I have some Thomastik flats on my jag right now.
Another string thats really nice are the D'addario half-flats which are rounds that have the tops ground off. They are a nice balance of both worlds. I think low budget flats take this route of taking a round wound and grinding them flat, where as genuine flatwounds start with a wrap that is flat to begin with all around and have a silk inlay between the core and the wrap. The flat thomastiks are really floppy and supple when handling them compared to roundswound strings.