TWOK was the only one in the set to have gotten a proper Blu-ray treatment, meaning it was scanned at high resolution from an original film. The rest are what I call "fake-ray" meaning they ran the file from the DVD through a sharpening software.
So I'm going to wait for a proper Blu-ray quality scan from film and director's cuts for the super-mega-signature-ultra editions, which we all know are inevitably coming.
except for STII:TWOK [4k scan] all the others were mastered from older 1080 scans and have too much DNR (Digital Noise Removal) applied to remove the grain but gives flesh tones a waxy look and also edge enhancement.
We can surely expect after 2016 a 4k mastering of all Trek films for archival and 4k releases in 5 years and a future Blu-ray re-release deluxe edition (in HD from the new 4k master).
It's your choice but $22. is a real deal for 6 films!