The Jerry Glaser screw chuck is nothing short of an outstanding product, one which I heartily endorse for my fellow woodturners. Go to http://www.glaserhitec.com/shop/glaser-screw-chuck/ to see more information about it but I will tell you here what I like and don't like about it. First, the don'ts. Actually, there are only two and the first is you must buy the screw chuck to fit your lathe. That means if you change to a lathe with a different size drive, you have to buy another one of these chucks, unlike four-jaw chucks for which you can get removable threaded inserts to fit a different lathe. The other thing is the price. These aren't free so you really need to use a screw chuck regularly to justify the cost. Even though the threaded barrel can only fit one drive size, it is very strong and durable so will likely work forever for you.
BUT, the utility of these screw chucks is amazing. The basic Jerry Glaser screw chuck, developed by Jerry Glaser who has now passed on, is one of the most valuable pieces of equipment I have in the shop. I have three, one for the 1x8, 1 1/4 x 8, and 33mm threads on my three lathes. The threaded barrel of the chuck body screws directly onto the drive spindle of the lathe and has a face which is about an inch and a half wide with a sharp-threaded screw in the center. Using it requires drilling a 1/4" hole in your work piece and then threading it on. The chuck comes with a detachable collar which threads onto the working end of the barrel to change the width of the face to about 2 1/2" or 3 1/2", depending on how it is screwed on. And, for merely more money you can also get an additional collar which gives you face sizes of about 4 1/2" and 5 1/2".
This is one of the most useful pieces of equipment you will ever have. Even though it is really expensive, if you use a screw chuck reasonable often, you need to give the Jerry Glaser Screw Chuck serious consideration.