I'm playing a Hexblade, started at 7th and at 13th I just multiclassed (long story). Obviously Xanathar's has made this possible, and I'm glad they changed some stuff from what was in UA, as the flavour and balance compared to other classes is better now (although I still feel that a full-caster gish is, potentially, too much). I've had a few players in my games play warlocks, all quite different, most of them multi-classed early.
You are kind of right, in that there's a heap of potential for variety with Warlocks, however from my experience that starts to fade a lot as you get higher level... you start to run out of interesting invocations to choose, and you start to look at the spell list and say "is that all I can choose from?" Case in point when I got 7th level spells, they were all pretty useless for what I had in mind, thank goodness at least that Xanathar's had one decent option although it wasn't really great for my particular PC.
Note: Tomb of Levistus isn't great, unless you love being able to say "wow, I would have been dead if I didn't have that!" - my Hexblade has had it for 8 levels, and used it exactly once ever, and he's one of around 2 "front line fighter" types in a group of around 4 PC's. But to be honest, that's my experience of a mid-to-high-level Warlock - you end up with a fairly large number of spells and other powers, but in reality a lot of them hardly ever get used, they are great to have, but just highly situational.
I tend to agree.
If your playing
Pact of the Blade your not going to have much of an issue with this because you can fight in melee most of the time waiting for the stars to align for one of your more niche spells or you can just
eldritch smite without waste to knock the crap out of a tougher enemy and not feel bad about it.
Shadow Blade (concentration) is an interesting choice for pact of the blade as an off hand weapons since it doesn't get an attack stat bonus to damage any way and it would give you a 3rd attack each round. It scales in damage but it can't activate eldritch smite and its concentration so it might not stack up to eldritch smite before you lose it... then again it might.
Pact of the Chain can play the eldritch archer with crowd control manipulation and pet tampering (quasit applying poisoned condition or aiding and ally for advantage on attack) waiting for that same key moment for a spell the way a ranger would not caring about running out of pact magic slots and still having plenty to do in and out of combat.
Shadow of Moil (concentration) might be the best pact of the chain spell because it lets you "hide" literally any where because of making you heavily obscured and surrounded in a 10ft radius of darkness. Which means your free to stand in the middle of the field and hide while you use your attack action let your invisible quasit
familiar use its reaction to walk up and attack enemies, putting poison status on all of them one at a time so all their attacks and saves are at disadvantage, and then use its action to turn invisible again so it can move around with a level of impunity. If you are found and attacked or simply decide to fight you have advantage on attacks and enemies disadvantage on attacks against you as a result of being heavily obscured and them treating you as if they were blind per player hand book errata. Then you can hide again as long as you
don't lose your concentration from being hit (which is why I don't recommend this for pact of the blade who will generally be in melee combat).
https://media.wizards.com/2016/downloads/DND/PH-Errata-V1.pdf
Pact of the Tome is the trickiest (in my opinion), you can take shillelagh and book of ancient secrets then play a master manipulator effectively using booming blade to give your shillelagh scaling damage or shadow blade and booming blade and focus on out of combat spells and invocations. Just don't think your equivalent to other full casters in combat, as you mentioned warlocks don't have the spell variety (known spells or number of spell slots at varying levels) to efficiently prepare and use spells in a continual and meaningful way like other casters. You will end up focusing on 1 or 2 spell likely with concentration in battle.
Most of which are not crowd control, high damage, or AoE in a meaningful way, the exceptions being Hunger of Hader (crow control AoE) and Enervation (damage). This means despite, and in part because of, the extra cantrips and ritual casting your likely to shine more out of combat where you can use you spells and invocation to manipulate recharge your pact magic spell slots regularly in which case the waste of casting a 5th level pact magic slot for 1st level charm hurts way less than it would in combat or Hallucinatory Terrain for a concealed while you rest which will restore your slots anyway.
Synaptic Static is perhaps the best Pact of the Tome spell since its not concentration, an AoE, has decent damage, and an awesome secondary effect... : ) makes you feel like a caster combine that with
Enervation and you don't feel as reliant on eldritch blast I think.
I have been playing a GOO Pact of the Tome Warlock scout with Devil's sight, One with the shadows, Agonizing Blast, Repelling blast, and a plane to pick up Maddening Hex invocation at level 9. I usually scout ahead of the group 30ft so I can telepathically report back what a head as I stop in the shadow of a corner and use One with the shadows to disappear. Also, I keep Thunder step handy so if I get ambushed scouting a head of the group I can leave a nasty surprise on my ambushers and actually teleport to relative safety behind my group to provide hex + eldritch blast ranged support (with bonus action maddening hex damage when I level up). I am not the best scout ever or the most powerful in our group for sure but Its fun to play and I feel I have purpose in the group.
I am enjoying the new Xanthar's spells and abilities for sure.
