As a relative KTM 990 noob, I bought a used set of KTM 37 liter aluminum side cases from an inmate back in January. Great cases for on-road touring (slim, the quick mounts make it a snap to get on/off at your motel/hotel/Holiday Inn
If you plan on any off-roading and like to get gnarly, those convenient quick mounts are pretty fragile...
Just ordered a set of Wolfman Expedition Dry Saddle Bags
for off-road/camping trips.
Lots of guys have the Gobi Cases
and love 'em.
Touratech sells the Zega Pros
And Jesse Luggage makes the super-beefy (and SPENDY!) KTM Jesse Cases
Many opt for various Pelican cases as both side and top cases (I'm still deciding which Pelican I'm using for my top case).
It all depends on your budget and what you plan on doing with them.