After hundreds of printing hours on my feeder the hook got a little loose, so I decided not just to replace it but also to strengthen it a little. Since using the U2RS means you don't have the size restrictions you would have at the backplate, I added a few millimeters here and there, and merged the body with a mounting plate. Also modified the hook shape a little to have more surface area touching the main arm.

My advice is to use any material you like (PLA, XT, ABS), but to use PLA for the body. If you have some wear on the body you might get some particles in your bowden/heater, you don't want ABS/XT in there when printing at lower PLA temperatures.

Search for "U2RS" to find other parts of the rail system.
printed - layer 0,1 - Fill 100% - speed 35mm/s
you need to move the knurled wheel a little towards the motor, just so much the fillament will be nice in the middle of it.

ultimaker2 rail system ;
https://www.youmagine.com/designs/ultimaker2-rail-system-u2rs

my original feeder ;
https://www.youmagine.com/designs/and-another-um2-feeder-design

Which was mostly based on the "mother of all UM2 feeders" ;
https://www.youmagine.com/designs/alternative-um2-feeder-version-two
edit 1/2015 ; Body update, added a little mass just around the bearing wheel, to have a bit more guidance for the fillament while using flxible fillament. Just did the UM extrusion test succesfully at 230 celsius to 8,5 mm3/s using (semi)flexible Makerpoint MP flex 45. Take this one when you want to use this feeder, probably no need to upgrade for most people using the V1.