It seems there is a difference in quality between episodes, although they are all quite impressive. I noticed some grain on the first few I watched randomly. But I just finished watching “Where No Man Has Gone Before” and it was stunningly clean and sharp!
In contrast I also just finished watching the first two Godfather movies and they are just about the worst quality blu-rays I have ever seen. From what I've read that was intentional. Supposedly. So you just never know what you're getting. Still I would rather have them as they were meant to be seen and not put through some questionable software alterations.
I wouldn't hesitate to buy the TOS blu-rays if the original poster is still debating, as long as the price is reasonable.