I went to Cornell last year. It was a lot of fun! All of the professors in the program were great and they chose interesting topics - accessible, yet didn't feel "fake" like many other undergraduate research topics. (My group is hoping to publish a paper in one of the better professional research journals, provided we can tie up a couple of loose ends.) Retrospectively I have the impression that Cornell is one of the more serious math REUs. I spent most of my time doing or thinking about math, at all hours of the day and every single day, including on July 4th. Some of my friends who attended programs elsewhere reported that their programs felt more like summer camps than a serious academic endeavor. /Unfortunately/ my success of getting into Cornell last year will be unrelated to my chances this year because the program was run by different professors.
I got into about 7 out of 10 programs last year. I don't have high hopes for Tennessee this year because they rejected me last year, despite being from a small school :(
As for general chances, my REU adviser told us that he received ~200 applications and that he picked us because something stood out about our applications. Someone asked him if he could share with us exactly what we did that made us stand out, because it might be a useful piece of information for graduate school admissions. His reply: "Oh, it's nothing you did. Your letters of recommendations stood out."