Because of the tendency to focus on problems with the game in this forum, here is what I like about turrets.
They generally seem to have a good range given their other stats (flak turret excluded of course).
Radar dampening on Mk. II and higher.*
Beam turrets are awesome. Mk. IV is downright scary strong.
Missile turrets fulfill the role of "squishy artillery" very well; I'm please with their performance.
The drone thing of the enclave is a nice bonus, even though I do find it hard to work into my micro techniques, or rather lack thereof.
Even though the basic turret is one of the weaker turrets, they are one of the most cost efficient too, both with metal/crystal/energy costs and knowledge costs.
Often times when getting higher marks of turrets, basic turret Mk. II is my first choice, and I then choose other Mk. IIs if needed.
I like their nature as the "cost effective but cap below average" they have. It gives them character.
Sniper turrets may be weak on the DPS, but their ability to, well, snipe across the planet's space is amazing, and IMO is enough to keep them very useful. They are a wonder for dealing with stray "stragglers". They also have a pretty nice targetting logic.
Not sure why they still have a very high cap though. Their individual stats aren't even all that bad compared to the more "mid-cap" turrets. Is it sort of to allow them a "set it and forget it" type thing for un-important planets?
Spider turrets are all of the awesome of the sniper turret, but with even better stalling capability thanks to the engine damage.
Tractor turrets feel like they are in a good place right now. Tough enough and effective enough to be able to effectively stall a wave, but not so strong such that they feel like they can just completely shut down waves.
When paired with grav turrets, the stalling capability is amazing. Especially if you throw in some spider turrets as well.
*I understand why Mk. I versions don't have it, being a "pre-unlocked thing", and I would like that to stay. It also serves as a way to give incentive to try the higher marks, though this may need some better "advertising" that Mk. II+ can provide this.