It is good two lane, four-lane, or four-lane divided in Arkansas, slower two-lane in Louisiana. Pretty lonely road, few services, from Arkansas line to I-20. Small towns will slow you down somewhat, and you have to watch for farm equipment on the highway once you are off the Interstate.
From Pine Bluff to the Gulf Coast it is flat. You will be on the Mississippi Delta, cotton and rice country.
After one trip down through Louisiana on US-65, I started crossing the Mississippi at Greenville rather than Vicksburg. The Mississippi side has more of what I'm looking for, places to stop, eat, get fuel, use the rest area. My usual destinations take me to Jackson rather than Vicksburg, but if going to Vicksburg I might be tempted to cross over at Greenville and take US-61 down. I've taken it partway, but not all the way to Vicksburg.
Other than slow speed limits, rougher road than Arkansas or Mississippi, farm traffic, and no services, there's nothing really wrong with US-65 through Louisiana. It is just that Mississippi seems to do a better job taking care of roads and meeting needs of visitors.