well, to answer your question instead of just point you in another direction, I have found very little difference in quality between any of the mandrels I have seen or worked with. Now, let me qualify that statement. The Mandrel actually only refer's to the straight rod with the threads and the 60% recess in the tip. A straight rod with threads and a recess is pretty easy to get right. The only real requirement for it is that it be STRAIGHT when you buy it. How long it stays that way is totally out of the realm of quality in the original part. The difference in most "mandrel assemblies" out there is in how the mandrel itself is held in the drive taper of the headstock. And in my experience there, it is more of a question of design features than "quality" per say. Some are held by a collet and nut machined into to the morse taper, some are held by set screws, both of which allow the mandrel itself to be slid in and out through the headstock to adjust the length that protrudes. Many people who have one simply use the appropriate collet in their Beall chuck to hold the mandrel. (this is probably the best solution, but expensive up front)
There are also some mandrels that are fixed length and which must use spacers to get the correct pressure on the bushings and tube being turned. This is a much less satisfactory solution.
And also, unless a 60 degree live center is used to correctly locate the end of the mandrel to the tailstock, it's not going to work right regardless of how it is fitted into the drive. Poor alignment of the tapers between the head and tail stocks will also cause poor performance of the mandrel.
And, just for the record, I also recommend turning standard tubes between centers rather than on a mandrel, but wanted to (hopefully) give you the information you were looking for. A mandrel and adjustable collet is still useful for turning closed end pens, fridge magnets, barrel trimmer sleeves, and other various and assorted tasks, so it don't hurt to have one around.
