We know that iOS is being rolled out to devices all over the world today, and that likely includes yours. Should you pull the trigger? Well, that depends. Below is a breakdown of which devices are eligible and our suggestions for whether or not to upgrade them after going through the appropriate preparations first.
iPhone 5s or 5c: Probably. If you purchased your 5s on a subsidized contract, you have another year (or more) left on it so upgrading is a good idea. If you bought your 5s off-contract, and you already pulled the trigger on a new iPhone, you may not want to bother since you'll automagically get the new version of iOS on that device already. Plus, it can be handy to have a previous version around the house for testing other things. Since these are the "one back" generation of iPhones (current until about a week ago), they are more than capable of running iOS 8. Specifically the 5s has access to all the new things coming to Touch ID, so as far as the software goes you aren't really missing anything.
iPhone 5: Probably. If you got your (subsidized) 5 on release day, you're due for an upgrade. This is very nearly the same hardware as the 5c listed above, so it isn't like you'll have a hard time running the new version of iOS. Again, things like Touch ID are a hardware addition, so since your phone doesn't have a Touch ID sensor there's no software change that will make that work for you. Otherwise you should be in good shape.
iPhone 4s: Think Twice. If you want to go for it fine, but keep in mind this is the oldest hardware that will run iOS 8. There are two big considerations here. First is the big screen: There's only three and a half inches of space on a 4s, and everything newer (5, 5c, 5s, 6, 6+) has at least four inches (diagonally) of screen. Some things like search results take up more space than they used to, so you may end up giving your thumbs a workout. And all that great stuff demoed at WWDC where your hardware is even better integrated like text messaging from your Mac, or starting an email or document in one place and finishing it in another, that's all called Continuity, and none of that is available to the iPhone 4s. So if you're planning to upgrade because you're excited about those features, it's not iOS 8 you need, it's a new iPhone. (Note that with the iOS 7 upgrade, the 7.0.1 update fixed a lot of things on the lowest level hardware, so if you still want to jump in, at least hold off until that update is out.)
iPad 4/Air/Retina mini: Probably. As of now, the Air is the latest and greatest tablet from Apple. This hardware is more than capable of running iOS 8, and it will look great on that larger screen. Generally people will buy a larger capacity iPad than iPhone, so if you have a 128 GB model, make sure you have the space available in iCloud or on your computer for a full backup before you pull the trigger on that update.
iPad 2/3/non-Retina mini: Think Twice. Read my advice for the iPhone 4s above, it's basically the same. While the iPad 2 is the oldest iPad supported, both the 2 and 3 are completely lacking in Continuity features. There's nothing wrong with that, but be aware if that's what you're looking forward to, you'll have to keep looking.