The Pex tubing is in my opinion great stuff. #1 it is both tough and flexible. #2 it can be a little awkward, but once you get the hang of it, it is pretty easy to work with.
You can certainly hang and pressure test it. I worked with it in cement (on the ground) so going over your head will surely increase the degree of difficulty, I can see having another set of hands being a real asset. Plan out your loops and try to keep them roughly the same length. Pressure testing is easy.
Talk to more than one plumber, discuss what he expects the system to do and how it will work, and go with the guy who seems to have experience and has an understanding of why he's reccomending what he is doing. There are pros and cons to everything involved, from the method of insulating, more or fewer zones, the controls, and so on. They guy who understands that there isn't one right way that applies to every scenario, and can explain why he thinks his compromises are the way to go would be the one I'd hire.
I would be far more concerned with a system that works the way you want it to than with leaks. Leaks can be repaired in a pretty straightforward manner, troubleshooting a poorly designed system that isn't operating the way you want it to can be a nightmare.
Edit: I'm sure you've already been there, but check
www.pexsupply.com the radiant install parts should give you an idea what the contractor is talking abour: plates vs. clips. You might want to check into whether a joist heating application is appropriate (I am unfamiliar with this stuff) for your circumstance so you have an idea of whether or not your contractor is suggesting the proper system.