The central air in my parents house is poorly designed, and some upstairs bedrooms get uncomfortably hot in the winter. Since those rooms are used infrequently, it's easier to either drop in a traditional window unit or use a portable than completely redo the HVAC to more appropriately suit the house.
After using a portable at their house for a few years, we switched back to a small window unit that was easy to install and remove as needed rather than the portable.
The good thing about the portable is that set up and take down is easy, as you just need to open the window enough to put the duct attachment plate in it. That still leaves you with a long(and hot) duct that needs to be dealt with and also reduces the efficiency relative to central air or a window unit. One other plus(and probably the only other one in my mind) is that they are nearly invisible from outside the house, as opposed to a window unit hanging out.
On the negatives-just in general operation they are a pain. All air conditioners condense water from the air, and it has to go somewhere. On central air, it runs into a house drain, and window units drip it out the back. Unless your portable is located somewhere where you can run it into a drain, you'll have to deal with emptying it(the frequency depends on how humid it is-around here in the summer getting it to run all night was marginal). Even worse, unlike a dehumidifier, I've never seen a portable A/C with a pull out drain pan that can be dumped. Emptying it was always a tedious process of draining into a low, shallow pan and dumping. It would easily take me 10 minutes to drain it, whereas I can do my dehumidifier in under a minute.
One of the other big issues with them is a bit more subtle. All air conditioners have to pass air over the "hot" side to transfer heat out(this is how they work). Window units and central air have ready access to outside air for this, and circulate this over them. Portables come in two configurations-"single pipe" and "two pipe", with the former a lot more common. The latter work basically just like a window unit except that the condenser coil air passes in through one duct, over the coil, and out a second one. A "single pipe" draws this air from inside your hose and pumps it outside. With the unit running, it's probably pulling more air than you realize-I'd actually have issues with running it in a closed room(even freezing up a few times) because it couldn't pull sufficient air. If you're supplementing central A/C, the air that you're pulling out is the already cool air from inside, and hot outside air is probably coming in through every crevice it can find.
I'd find that even though the portable was meant to cool one specific room, it could often negatively impact the rest of the house and/or make the central air work harder. This is in contrast to supplementing with a window unit, which basically has no impact on the central air.
My suggestion-if you can do it-just get a window unit. They're less expensive, more reliable, easier to operate, and just work better.