We love the NL 3600 for a variety of reasons -- ringspun, soft hand feel, tearaway tag allows for value added upsales to custom tags, etc.
The only downside I've seen with the NL 3600 (and the BC 3001) is that it's pretty lightweight cotton and if it isn't taken care of properly in the wash, we've had some customers complain after a month of washes that it starts to fall apart a bit.
I have some NL 3600s I printed 2 years ago or so and I still wear them and they still feel fantastic. I'd say this is my favorite tee under $4 for DTG.
Another possible downside is that the cotton is so thin you sometimes can get ghosting on the back side if you're going heavy on pretreat and ink. It isn't necessarily visible but when the shirt is wet you can definitely see th ghost impression.
