There hasn't been much C+ info here about the A321 since it was added to the Delta fleet.
I flew in one yesterday, and offer some observations (I also added an entry to the WiKi up top. As time permits I'll go back and see if I can find some earlier seat comments).

-- 13EF: I sat in 13F and partner in 13E. We thought it was great. These seats are somewhat narrower than the standard Y/C+ seat, but not bothersomely so (and I am not by any stretch petite). Legroom is excellent, and there is not a slide in the door. FAs on our flight had no issues with properly stowed smaller bags under 12EF, though you can't really reach them when belted in.
IFE screen (w/ USB) is armrest mounted swing-up, so must be stowed below 10,000ft. Tray tables are in the armrests, also (slightly) reducing seat width. 120v outlet and second USB under the seat. Window view is OK, by the right shoulder but not neck-craning.
FA jump seat is set forward several inches so 13E has good left elbow/shoulder room. It felt very private for a couple. On this new airframe, door seals are still good, so not cold/drafty/noisy. Note that the o.h. bins run along the whole C+ cabin, so both my partner, and someone across the aisle hit their head standing up (assuming, I think, that an exit row with this large-ish door shouldn't have a bin over it).

-- 13ABC: This triple is set forward a bit more than 13EF, still excellent legroom (and no boarding at this door). Window view from 13A is more of a neck swivel. PTVs/trays are in the armrests. Seatbacks may be a bit narrower because of the thickness of the door vs regular fuselage section/seat width in rows 10, 11, 14.

-- Row 12: Seats 12A/12F I would rate avoid, from how crammed the pax in front of me looked. No window, and they have very narrow seat backs due to the fuselage space taken up by the exit door frame - These might be the seats the DL web site lists as being 16.3" wide? It looked like 12BC/DE were somewhat narrower than standard. Since the std A321 seat is 18", 12BC/DE (and 13EF) might be like a typical 737 seat - guessing 17.2" or so. I'd say 12C/D are acceptable aisle C+ seats.

-- 14ABC: Looked excellent. Didn't get to try them, but because of the offset of 13ABC, the pitch looked extra generous, and nice window alignment. 14DEF should be standard C+ seats.

-- Row 10: Didn't get much of a look, but there is just the F cabin little divider below the bins, so leg room should be good. Didn't notice anything good/bad about row 11, but didn't linger to really look.

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A few other A321 cabin notes & thoughts, not C+ per se:
- 26BC has no window (just the tiny porthole in the door). I think 27A has an OK window view, and of course excellent leg room.
- 39A has one window, but fairly far forward. At least it didn't appear like the sensory deprivation zone of the new last row on the A320.
- The 'spaceflex' lav at the tail that is closer to the galley is tolerable. It seemed slightly bigger than the similar positioned lav on the modded 320, which was laughably small. My partner described the lav at row 25 as a tiny port-a-potty, I didn't try it.
- We happened to connect to a 75D on the same trip leg. I've always been fond of the 757, much more so than the 737. The 75D interiors (despite mods) are showing their age, especially on the PTVs, and the tighter seat-set widths and lav in the middle of the 75D C+ 'cabin' make the 321, in my first ride anyway, a more pleasant experience. I hope DL eventually orders some of the longer range 321neos, frankly. I could see doing a transcon, Hawaii or short TATL in C+. Certainly better than a 739!