I'm remodeling a bathroom, and some of the copper tubing needs to be replaced, due to leaks and the need to reposition fixtures. Is it feasible and recommended to convert from copper tubing to PEX? For instance, could I use something like this where the copper comes out of the slab, and run PEX the rest of the way to the fixtures? My motivation is (perceived) ease of working with the PEX, especially the ability to position it where I need to for the shower valve assembly. I say perceived, because I've not ever worked with PEX before - is that asking for trouble?
Yes you can do that (it's the standard way of converting to/from copper), and it shouldn't cause any troubles (read Why would you use copper over CPVC or PEX? and Does Pex tubing have any advantages over copper? for some comparisons).
Of note: you can get the male adapters that fit on the outside of copper pipe, and also that fit on the inside of copper fittings (like a street elbow). Can be handy sometimes if you're working in a tight space, or were going to immediately make a turn anyways (since you're soldering the fitting anyways, it's barely any extra work to do an elbow+adapter, and saves a PEX elbow and couple of crimp rings).
If you want to make this task really easy, you can use a sharkbite. Its a connector than can join different tubing without the need for soldering. While expensive for a connector, ~$5, it will save you the hassle of sweating pipes, especially if they are in a hard to reach area. You can find them at HomeDepot, not sure what other stores carry them.