I have a 12,000 btu LG portable that I've used two seasons as an emergency A/C for some townhomes I own. It has worked very well. A friend used it for two weeks last summer, daily temps were 100+. He had to replace his central H/A unit, and while waiting for parts he used my LG. It kept his living/kitchen area cool.
I found mine on Craigslist for less than $200, so it's worth the money. But, I have heard some folks haven't had good luck with portables. It is easy to hook up, and where the vent goes into a window opening I use a piece of coroplast to fit the opening and then cut my vent opening, duct tape in place for the temporary use.