It was monotonous. Warlock was my first 5e char, played levels 2-15 over a couple of years. The DM would rarely give us short rests even when we asked/RPed for them (once spent 8 hours on an airship where we were allowed to do "downtime activities" and yet I was not allowed to SR). So yeah. It's the class whose combat casting is completely at the mercy of your DM which is why players have such wildly different experiences playing one.

I, as I imagine most people, always want one slot in reserve as a clutch backup. I think I tried casting Hex in a fight once, lost Concentration and felt so shitty about wasting an entire spell slot on it that I never cast it again and stuck to only Armor of Agathys if we had pre-fight prep time. I basically became an out-of-battle caster. Once I got the third spell slot and I felt I could "risk" two slots - that became a LOT more fun as I finally felt like I could use the Bonus Action Concentration spells (Hex, Far Step).

"Eldritch Blast, end of list" was my entire turn for like... 7 levels. It was boring even though numbers went brr, and with a Wizard + Bard (sometimes Sorcerer) in the party I wasn't the versatile spike spellcaster nor was I needed for my support stuff like Dispel Magic or Dimension Door. I was old reliable EB machine gun with a Pact familiar who would get looked at wrong and die so I also felt I couldn't use him for advantage in a fight. I didn't feel like I was really filling a niche we needed until I got Scry at level 9 (never had a Cleric, Bard had swapped to Sorcerer character at that point).

I love this class, you just have to know what to talk to your DM about beforehand. Expanded Spell Lists should just be known, too!