It's the same material and process as HREW tubing - providing it is 'proper' ASTM-53 pipe. Which is pipe which is ment for structeral applications. Not just fluid flow.
A lot of people knock it. And they can do that - I won't argue with them about it. At the same time there's nothing wonderous about it - it ain't no chromoly DOM.
I figure for the amateur cage builder the design and weld quality far outweighs if they used pipe vs conventional or high end tube. Providing they don't do something stupid like use .75" sch20. But I consider that to fall under design.