For hobbyist usage, a Yongnuo is probably a good choice. But you have to rely on the retailer selling it to you for warranty/service, because direct communication with Yongnuo can be hit and miss. Most of us wait until you can find a "fulfilled by Amazon" unit, rather than just purchasing directly from China or off eBay.
The cheapness has to come from somewhere, and it's typically in terms of QA, service, warranty, and copy/component consistency. In the early days, a number of new units would be DOA, or had faulty capacitors that blew very quickly. For the handier and more electronics-fluent folks, it was no big deal to unsolder and replace the capacitor. For others, not so much.
They eventually figured out the issue and those type of DOA issues are rarer, but with newer models, a lot of early adopters may inadvertently become beta testers. For example, the new YN-600EX-RT has a lot of folks reporting about battery drain issues, dead lines on the LCD, etc. etc. As a Yongnuo shopper, you become very familiar with hidden versioning and how to find those stickers with the production date on it.
That doesn't mean you'll get a lemon. Most folks who get Yongnuo gear don't. But you don't exactly have the recourse for dealing with a bad unit as you would if you bought OEM. And because the technology is reverse-engineered, both backwards and forwards compatibility is not a solid as with Canon gear. I have a YN-568EX that works great on my 50D and 5DMkII. It won't even fire on my Powershot G9's hotshoe. My 580EXII works fine on all three, and can even have its power level controlled by the G9's menus. The compatibility of Yongnuo flashes with 1-series cameras or film cameras is again, less. A YN-600EX-RT is a great unit at low cost, but it's not identical to having a Canon 600EX-RT: you can't use it as an optical master, it has no external sensor, etc. Resale value on a Yongnuo is nowhere near that of a Canon. If you are a professional shooter who stresses their equipment with heavy usage, then a Yongnuo may also not be for you.
As long as you understand that you're not getting an exact equivalent of a Canon flash, you'll be happy. But my advice tends to be that for a first or only flash, if you're going to be using this flash heavily or plan on keeping this flash through your next three camera bodies, consider getting used OEM gear, or at least a 3rd party flash that allows for firmware updates (Metz, Nissin, Sigma, etc.). Yongnuo becomes a no-brainer for your 2nd, 3rd, and 4th flashes in a Strobist setup. Especially if you go cheap-cheap and minimal-feature with the manual-only flashes, like the YN-560III/IV, where there's less tech to go wrong.