I have a lot of experience long reining, initially with the purpose of starting youngsters. I believe it is a huge tool to have and I have spent many hours over the years developing and practising my skills.
Lunging or long reining are both an alternative way of exercising your horse without the weight of a rider on board, which may be a requirement of rehabilitation, or just a way to bring in variety to your exercise program.
I will always advise long reining over lunging as I believe the lack of control lunging can prove hugely dentrimental to any horse. Lunging also relies on 'gadgets' to encourage a horse to work correctly but either leave the horse fixed in a certain position (e.g. side reins) or allow them to bend or flex incorrectly to avoid connecting.
Long reining allows not only better control, but adjustability and the ability to feel through a contact like you would from on board.
It also allows you to see things that you may not feel from on board. Does your horse track up correctly? Are there differences in stride length or foot placement?
Like ridden work, long reining can be developed to include pole work through all gaits, although this should be done progressively and to suit your horse's abilities.
I offer long reining lessons as well long reining myself. This is not an easy skill and takes time to develop. Not all horses accept the lines so it must be done in a secure school until the horse is happy, relaxed and under control. Once that point is reached long reining can happen anywhere!!!