With sumps, you'll have more of a DIY situation rather than an off-the-shelf product in most situations. I think it's great, because you can tailor the setup to what you want. There are commercial sumps that can be purchased, but the cost is much greater and there are often fewer available configurations as a result.
A sump would make sense for a display tank where you want to keep all of the equipment out of the display tank.
A sump would also make sense for cichlid tanks where it's hard to over-filter with conventional equipment. The larger the tank, the more likely this becomes the case. I say starting around 180 gallons and up, though some of the newer canister filters are quite capable (FX6 can hold >5 gallons of media, etc).