Are super cheap flashes worth it?
In certain usage situations, absolutely.
- If you are a hobbyist and don't need rock-solid reliability or plan on super-hard usage
- If the cost-savings is worth entering a copy/component quality lottery
- If you don't mind being an early beta tester with new models
- If you don't mind Chinese reverse-engineering issues (future/backwards compatibility)
- If this isn't your first or only flash
- If the feature sets fits what you want to use the flash for
Super-cheap flashes are typically reverse-engineered technology of Chinese manufacture. The low-lost cost has to come somewhere, and it's mostly from the QA. While this has gotten better over recent years with Yongnuo, there's still a larger variance in component and copy consistency than with OEM gear, and warranty service isn't nearly as accessible or reliable. A Yongnuo flash must be mailed to Shenzhen China for repair/replacement. The postage alone typically means most folks will just replace a YN-460/560 rather than get it repaired if it's faulty. They're often seen as disposable units.
And if you need to have a working unit at all times, pros tend to get multiple copies of Yongnuos, or shell out more bucks for, say, a LumoPro or OEM (Canon/Nikon) flash they know will be more reliable or at least can be serviced in the US.
The second thing to keep in mind with super-cheap flashes is that they tend to be manual-only with a bare minimum of features. They can really fill the bill for a hobbyist learning to go off-camera flash Strobist-style as second, third, and fourth flashes. But for a first or only flash, where you may want to use the flash both on and off-camera, having TTL (auto power setting based on metering) and high-speed sync (HSS) in at least one flash can be worth the additional leap in the pricetag.
no radio trigger.
Even back in 2012, this isn't really an issue, since radio triggers have become really cheap and easy to add to a flash's foot.
In 2016, it's far less of one, as receivers are now being built-in. The YN-660/YN-560III and IV all have built-in radio receivers that even allow for remote power/zoom setting. This is becoming more and more widespread with all flashes (even the OEM ones). You do get locked into a specific trigger system, though.
It doesn't have i-TTL I didn't really know how it works, but I guess I can have better control and possibly better results just setting all the things by myself. How complicated is it to do calculations and setup? Do I need to measure distance of the subject for it?
It's easy to do if you or your subject aren't moving, and you can reshoot a couple times (studio conditions) and have time to adjust. Very hard to do if you're shooting an event, are moving in and out of different lighting situations with a moving subject, and only have a split second to dial in the power. This is why TTL and prior to that, autothyristors, are built into more expensive flashes to automate setting the flash power. It all depends on how you plan to use the flash. Think of having iTTL on a flash like having A mode on your camera--very convenient for fast operation. You use iTTL on a flash for speed, and M for precision.
In addition to this, iTTL also means the flash can communicate with your camera to do things like FP sync (if your camera body supports it--the D3xxx and D5xxx bodies don't), so you can use flash with a shutter speed above your body's maximum sync speed, and CLS [optical remote flash triggering with full hotshoe communication for remote power setting, 2nd curtain, FP, Zoom etc.]
See also: What features should one look for when selecting a flash? and What are the Yongnuo flash naming conventions?