Once you reserve a jumpseat, what is the priority as far as getting bumped out? Would other UPS employees or the FAA have the only other priority? Also, i am unfamilair with riding on cargo carriers, are there many cancels do to crew/mx/wx issues like we see in the airlines? Ive heard people say that if the flight is scheduled, it is going. Thank you.

Well, once you are listed, there is always the possibility of getting bumped, it just depends. We do not have any kind of "lock" on the seat 24 (or whatever) hours out like FDX does. If the company needs to use the jseat for a higher priority jseater at the last minute, they will bump you. You essentially have the same priority as our own pilots do when traveling on UPS acft on their own.
Call the jseat desk number that you originally booked the jseat on the day prior or just before the flight and ask them how many seats are taken/how many are still open. As long as the flight is not fully booked you (should) be fine.
As far as wx/mx, etc..., it's no different than the pax carriers. The main difference is that we generally are more on-time (not waiting for pax as much) and it is not uncommon to have flights leave 15 minutes or so early if the planes loaded and ready to go.
Where are you trying to jseat from/to?

Actually the priority is thus:
1. Li'l Stevie
2. Other UPS employess
3. FAA

You'd think one little felt puppet wouldn't be that much of a concern for being bumped, but I hear he takes every chance he can to get close to a MCDU.

It really depends on where you are going. If you are trying to go ANC-SDF; they get very full due to commuters. The jumpseat desk should be able to help you out with loads. Where are you traveling and when?

Well someone had to say it. Good one

UPS has always been very very good to me. When bumped, I have been called by UPS, so for me it wasn't a mystery, I knew beforehand (doesn't mean it will happen every time). If you can't make it, be sure to call and cancel as well.

Everyone was always very friendly and very awesome, it's was certainly a privilege to j/s on them. My QOL went up several points when I started commuting on them regularly.