I have Comfort and use it regularly. It's very accessible, and would be useful even to someone who knows little Greek. His discussions are helpful.
Here's what Comfort has to say @Rom 8:28 –
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Metzger was on the committee for the NA27th, I believe, and discusses the reasons behind the decisions. I use his commentary regularly, as well, but Comfort is more helpful to me.
Both Comfort and Metzger deal with major variants. They are not comprehensive.
There are variants listed in the CNTTS that are not found in the NA27, NA28, or UBS4 apparatus. Additionally, the CNTTS has much more thorough descriptions of the manuscripts. It cites only Greek and and Latin witnesses – no church fathers or other versions at this point. I use it almost daily because it is so thorough, but the amount of data can be overwhelming when you first start using it. It also takes some practice 'decoding.'
I agree with the others that Comfort is the place to start. If you're interested in textual criticism, you are going to want to own them all eventually: Comfort, Metzger, NA28 Apparatus, UBS4 (5th edition is coming soon so you might want to wait on that one), and CNTTS.
If your Latin is decent, you might even want Tischendorf's Apparatus. It covers some territory that the others don't. I purchased it, but my Latin is so rusty (it’s been 45 years), that I returned it. If a translated version (Latin to English) ever appears, I’ll buy it again.