So we started playing with 3D printing last year after I built a Linear Prusa REPRAP. Didn't get a lot done for the robot but we did build some slides and electronics mounts. We also printed a ton of wrenches to hand out at competitions.
I taught a 3d Printer class (where everyone built their own printer) over the summer which was attended by both members of our team and some other teams.
We now know a lot more and know what we can print if we design it so I hope to see it used more this year with not just FRC but also potentially FTC.
That said to add to what folks said above - Calibrate, Calibrate, and Calibrate.
From a cost basis the REPRAP kits are affordable and from what I have seen if carefully assembled and CALIBRATED can print things as good as the lower end commercial models.
Plus if the kids do the assembly and calibration it is an opportunity to learn about a lot of different concepts.
Personally, of the REPRAP designs I currently like the Prusa I3 although there is a certain minimalist elegance to the Linear Prusa V2 which is made up primarily of Rods and printed parts. I have sponsored a Kickstarter which should deliver me a good Rostock (delta style) printer later this year.
A very reliable vendor of Kits who makes really quality stuff is MakerFarm
(
http://www.makerfarm.com) and he offers both V2 and V3 Prusas kits at
really good prices.
There are lots of different host, slicing and firmware options out there. Look up the online version of MAKE magazine which recently reviewed a number of slicing programs. There are also several electronics options but not all firmwares will work with all electronics.
Currently my favorites are:
Electronics: RAMPS 1.4
Slicer: Slic3R
Firmware: Marlin
Host Program: Repetier
I am looking to offer the 3d Printer class in the Northern Virginia area again later in the fall. Contact me directly if interested.