Depends literally how you use them. I don't like carriers (any of them) but thats because it descends to me swearing at pets. However used right? Yeah carriers work. They are not a waste of a ship slot; anyone who thinks that just can't use their ship!
Vo'Quv is slow but still useful. That Kar'fi though is extremely good. Quite literally again, how you use them.
Quick Edit: No, literally can start spaming out bird of preys with a vo'quv if you so desire. Nothing like scorpion fighters.
I have one of the most coveted ships in the game, the Jem'Hadar attack ship. it's dressed to the nines too. (has all the mk12 gear) When i got and fine tuned the Jem dreadnought carrier, i like it much better, I;m in the carrier most of the time. How good is it? Great fun! someone else actually named it for me. it's called the monster. Its just a different style of play, i am a tac captain, i use escorts and their variants a lot, but i also like the challenge of trying something different, it all in what you like. give it a shot in fair time, a month or so, you'll like it for the change o pace.
Carrier pets are slottede as equipment, like weapons or shields. You gain an ability on your tray to launch them, and you can do this an infinite number of times limited by a cooldown. They do not "wear out". Different types of carrier pets can be chosen for pure damage (quite a lot of it), target neutralization (tractors, energy siphoning), or even defense (shield repair). A hangar bay is a flexible and very powerful thing, and ships with hangar bays come with a variety of other capabilities. Escorts-with-pets like the Armitage or Jem HEC play differently than the heavy carriers like Atrox and Vo'quv that trade agility and some direct firepower in order to launch twice as many pets.
Soooo.. Are carriers good? I'd say yes, absolutely! Once you get the hang of them anyway. It does make your life somewhat more complicated since you have more things to keep track of.
Carrier operations are a very different playstyle from any other ship class in the game.
On top of that...there are different carrier types that also offer additional playstyle options.
Carriers can be classed into three general categories:
Pure Carrier (Vo'quv, Atrox, Jem Dread) -- very large vessels with a low turn rate.
Escort Carriers (Jem HEC, etc.) -- small to mid-sized vessels with decent turn rates.
Flight Deck Cruisers (Orion) -- large vessels with cruiser turn rates.
Pure Carriers rely mostly on their flight wings to apply damage -- this means you need to spec for carrier-specific roles, to include your space doffs (hangar bay cooldown, etc.). The carrier in this role is a support ship for the fighter wings.
Escort carriers fly into battle alongside their fighter wings and are damage dealers to some extent -- although they typically cannot do the same amount of damage as an actual escort class. Escort carriers cannot launch as many fighters as a true carrier, and the types of fighters it can launch are limited to the smaller types.
Flight deck cruisers fill both a light carrier as well as a cruiser role -- they are strong vessels with high survivability...and decent damage output. Like escort carriers, they cannot launch as many fighters as a true carrier.
Space combat in STO differs from player to player -- those players who fly escorts are going to look at carriers as a waste of time and ship slots because they don't entirely comprehend how powerful a real carrier -- when properly used -- can be. Although Carriers have been nerfed in the past (number of wings that can be deployed cut down, engagement range for launches reduced, fighter damage (esp. torp damage) reduced, and hanger bay recharge times increased...) they are still powerful combatants that can be game changers -- this is especially true if you are in a fleet action with more than one carrier on your team.
In PvE Carriers are very viable and can be fun to play if you like that playstyle. I have not PvPed in a while...I have flown a carrier in PvP in the old days (a Vo'quv) and it performed well...but there have been a lot of changes since then, and so I do not know how effective they might be today. My impression of PvP in the game at this point is that it tends to be escort-heavy, with a lot of fast-moving alpha strikes...for carriers, that is not an optimal environment. Carriers are most effective (I think) when going after larger targets, like cruisers and borg ships, for instance...
Bottom line -- carriers can be a lot of fun to play..but it is more of a personal choice, and the playstyle is very different from your "cloak, prep, strike" type of tactic.