As said, the simple answer is "just install it and don't worry."
Don't read too much into my comment, but the problems you've heard about are that Flash is, compared to the browser itself, relatively unstable and tends to be a CPU hog due to badly-written Flash-based ads. The former is just an annoyance, and the latter is mostly an issue on laptops, where some people notice the fans being noisier and decreased battery life.
Neither is really that much different than on Windows--most of the Windows users I know have about the same things happen to them.
The easiest solution to both is to install one of the programs that causes Flash to only load when you click on it (you will see a grey square labeled "Flash" otherwise). That means you only have it running when you want, and it also decreases the number of annoying animated ads trying to grab your attention when you read news websites. Which one specifically will depend on the browser you're using--FlashBlock or ClickToFlash are a couple (Google or search on
MacUpdate.com to find them).
(Full disclosure: A lot of Mac users, myself included, have a burning hatred of Flash due to past history and some current issues, but I'm trying to be straightforward and objective here, since a lot of people don't even notice the issues.)