Very. They don't mix, so a warlock/caster has the spell progression of both, separately, but they can use their slots for any spell they know.
So 3 levels of EK would give him 2 1st lvl spell slots that recharge on a long rest, while the Warlock gives him 2 increasingly powerful spell slots that recharge on a long rest. At lvl 5 Warlock, lvl 3 EK, that means he's got 2 1st lvl spell slots that recharge on a long rest, and 2 3rd lvl spell slots that recharge on a short rest. He can use the Warlock slots to cast during battle, and save the 1st lvl for long duration spells or those he only wants to do once a day, but he can use any slot for any spell he knows.
It gets complicated but having some slots that recharge on short rest and some that recharge on long can definitely be an advantage, especially for Sorcerer/Warlocks, who can convert the Warlock slots into Sorcerer points, thereby granting him a good way to recharge his Sorc points on a short rest.
Edit: I love Warlocks. My favorite characters have always been warlocks. I've tried almost every patron. Still need to try out the Hexblade, though.