Does the job I got it for, which is joining MP4 files. More specifically, to add 5 seconds of blank space to the start of each video. I created a 5 second video containing a blank picture and silent audio track, but with 500 videos to process, I didn't want to have to re-encode every one of them. So I encoded the 5 second video to match the MP4 files I was working with, and tried this out, and it works perfectly.
A couple caveats, however, if you are joining files and using the queue to do so, like I did.
One, if your destination filename contains non-standard characters, like "à" (accented "a") or "—" (long dash), the program will give every indication that it has succeeded, but the destination file will simply not be there. Oddly enough, it did not have a problem with a "ü" (a "u" with an umlaut) in the filename. Easy fix was to change the offending letter in the filename, and then correct it in the new file once processing is done.
Another, much more troublesome problem, however, is that if you run out of disk space on the destination drive (in my case, an external hard drive) before the queue has run out, the program will continue to run, but - for obvious reasons - there will be no output files. It would have been nice if there had been some kind of error checking in place that would have halted the queue and said something like "hey, stupid, didn't you check to make sure you had enough disk space?" But no, it just kept going, processing 255 of my files in vain - I estimate around 2 hours of wasted time for my computer. At least I wasn't just sitting there doing nothing and wasting my own time as well.
In all fairness to the developer, these both may have been flaws in the mp4box program itself, and not in this GUI. Nonetheless, the problems exist, so if you use this GUI, you may want to be aware of them. I know for a fact that this was the case with the first problem, and it has been corrected in the latest version of mp4box, but they also changed one little parameter (one that would affect me, anyway), so you can't simply copy the new version of the program into this GUI's folder and expect it to work (I know... I tried).
Review by awgie
Aug 19, 2015 Version: 0.6.0.6
OS: Windows 8 64-bit Ease of use: 8/10
Value for money: 10/10