**EDIT** BILLVO beat me to some of this already... .
Is the test difficult? No,
if you study, are an above average driver, have a basic idea of how a truck works, and can memorize facts/figures.
The written tests are pretty much standardized nationwide. The basic things like following distances, air system operation, and identifying certain parts of a truck and how they operate, etc.
The real test is the driving... ... a good driver can make it look easy, but the most important things I can tell you to keep in mind:
1) pay attention to your shifting--don't grind the gears too much, #1 give away on seat time is how smooth you shift up AND down... mostly down FYI.
2) be aware of how touchy properly adjusted air brakes can be. Last thing you wanna do is faceplant the tester into the glass--and it's very possible to do that with tight brakes on a light (unloaded) tractor/trailer combo.
3) Pre-trip walkaround. There are 50-some points to check and you're only allowed to miss IIRC two or three at most. The list is in the manual and you need to have it memorized, no cheat sheets to go off of.
What you do on the drive test will vary depending on your instructor, but be prepared to back around a (relatively) blind corner, stop as close as possible to an object you can't see---taking your test in something other than a 379 pool table hood Pete helps in this area, LOL--and turning a corner that, if executed perfectly, is
juuuuuust wide enough for the combo to fit around, and in general be sent down roads you don't know with a truck nearly too big to be there in the first place. Watch for tricky traffic signs/signals, turn lanes, etc... . instructors like to gauge how well you know how to drive a truck by forcing your attention off the truck and onto the road/traffic around you.
Most instructors (that I've been around) are either truck drivers themselves who do the instructing for extra money, or paid employees of the state dept. of transportation. Their methods will all vary, but the professional truck driver instructors will usually let you 'cheat' in a few areas---shifting w/o the clutch, shifting w/ the jake, not downshifting for every stop, etc. Learn how to shift both by floating the gears and by double-clutching so you aren't in for a surprise come road test day. Many of the professional 'testers' go by the book, and float shifting technically is a no-no.
If you have time to practice it won't be so bad, but remember too that heavy trucks aren't like cars & light trucks---each one is different and has its own quirks--there are so many engine, transmission, differential, suspension, and truck model combinations out there that it's best to take the final drive test in the same truck you practice in, so you have some 'local knowledge' of how it operates. Nothing like learning how to drive in a nearly brand new truck w/ AutoShift, air ride, and a nice smooth electronic engine, then go take your drive test in a 25 yr old wore out lowboy with a super stiff throttled turned up mechanical 3406, a transmission missing half it's thrust bearings, 5. 13 gears, and spring suspension to bring you back to reality. :-laf
Hope this helps,
Dan
Attention: TDR Forum Junkies 
:-laf
