Cassini also spent several years exploring Saturn system, but discovery of geysers on Enceladus and rivers/lakes on Titan only makes them more interesting.

Gas giants by itself are less interesting. Sure they will photograph those enormous clouds up close and measure their oxygen/hydrogen ratio with more precision, and maybe that's exciting for some scientists.

But you look at Europa and Enceladus and can't help but imagine rovers driving there one day, and can you imagine some craft floating down the river on Titan shooting 360 video?