I would say that the .3 nozzle is great. I started with the .4 hexagon nozzle and switched to the .3 to see if smaller details could be improved. I think it definitely does this, which is great. Higher detail and more threads of plastic on finishing layers, giving it a slightly smoother finish.

The only negatives I've found is that when trying to get a good slightly squished first layer, there's .1mm less plastic of play (room for error) getting the first layer to stick but not press the nozzle to the glass. Simply, what I mean is that it's a little harder to tune in your first layer. Some may argue that a .3 nozzle causes your prints to take longer though I wouldn't say there is any noticeable additional print time compared to the .4 nozzle.