Dan
Should You Practice With a Golf Net?
A routine question regarding the best and most efficient way to practice involves the use of golf nets.
When used correctly, they can be a great training tool.
When used poorly, they can actually keep you treading water, or even potentially groove some new bad habits into your swing.
Understanding Feedback

To understand how a golf net can be helpful (or hurtful), it's important to first understand the concept of feedback in the golf swing. Feedback is anything that helps you understand whether or not you swung the club "well".
A slow motion video of your swing, swinging in front of a mirror, or even watching your ball flight to see if it slices / hooks / pushes / pulls are all ways to give yourself feedback on your swing.
Any combination of these feedback mechanisms will help you understand most components of your swing, such as whether or not you moved your lower body efficiently or the angle of your club face relative to your path at impact.
How to Use A Net
With the concept of feedback now established, it's easy to understand how a net might help (or hurt) your game. Since using a net prevents you from getting feedback from the ball flight like you would at the range, you have to use a mirror or video to give yourself an understanding of how you're progressing through your swing movement. Absent that, you're just "swinging blind".
Final Word
Nets are great tools for helping you groove a swing movement, so long as you are focused on the movement itself and use a feedback tool like a mirror or slow motion video. It can be nice in the early days of working on a new dimension of your swing to get reps in without worrying about the ball flight.
However, don't just blindly crush balls into the net without stopping to analyze how you are swinging. With no feedback at all, you're bound to develop bad swing habits.