Hi All
I used to work for Tein back in the day and the guys there are sound. They have a lot of belief in their product and support it very well. You can guarantee the fit will be spot on, and they are rebuildable. They have worked hard to improve over the earlier products sold exclusively in Japan. i.e. the coating of the threads.
Spares were available separately and there is a strong network of dealers.
They were one of the first to come up with a completely threaded shock body. The fact they are so affordable is a result for customers.
Despite these benefits, they will not last for ever. They will need taking care of more than standard suspension. i.e regular cleaning, lubrication.
If they are to be fitted and forgotten about, you would be better off with springs.
My 2pence worth