As above really. Ever see the nature documentaries where the squid come from 1000's of meters down to the surface to feed at night? That kinda thing can be hard to replicate in an aquarium.
Easier to just keep cuttlefish or an octopus in my opinion, as they still display intelligence etc, but are actually doable.
There are some really small species of squid that work, but I am not aware of their being publically available. Most squid species are schooling, and really would need to be housed in HUGE groups to be happy. That is hard to do in an aquarium.
We have some in a lab at school. If they are not housed in groups of 30-40 we have problems. We pull them apart, breed them and put them back in their schools as soon as the eggs are layed. The babies need to stay together in a HUGE group until they are nearly full grown or they die.
any cephlapod requires special setups and squids would be extremely costly, and not worth the time or money considering they swim erradic patterns and flow would be almost insane as for cost and pumps required.
Check out this month's issue of FAMA. There's an article on keeping cuttlefish.
Largest drawback I can think of is their longevity. None of the species kept in aquaria are long lived. Figure only 12-15 months until they die of old age.