What do you like best?
The thing about QEMU is that it has so many things to like!
First of all, it emulates pretty much any CPU you can think of. x86, ARM, PowerPC, and even some obscure RISC chips and custom chips. CPU emulation is stable for all platforms you can run QEMU on, and most systems run flawlessly.
Operating system support is fantastic as well, with Windows, Linux, and even Mac running fairly well. Other systems, like Unix and OS/2 and the like, run great as well. Especially considering that before QEMU you couldn't really run any of those systems well on Windows or Linux.
QEMU on Linux can run Windows at near native performance with VFIO, KVM, and a few other technologies switched on. If your CPU supports hardware virtualization, expect at least 95% of performance consistently.
QEMU can not only virtualize environments, but can also do things that make using QEMU easy for beginners, things like hard drive image creation and disc image creation. There are probably 1,000 reasons to use QEMU (like Xbox support with XQEMU), but for now I will just say it is probably the best system virtualization package available at the moment.
What do you dislike?
The only real complaints about QEMU that I can think of are all related to the Windows version.
First, there is no GUI that comes with QEMU on any system you run it on. This means if you want to simply emulate or virtualize something real quick to show a friend, or if you are a less tech savvy person, you may not have an easy time using this software.
Another issue is that KVM and VFIO don't have support on Windows. This slows down the Windows version considerably, and while you can still use most systems fine, Windows 10 or Mac OS X 10.10 will run fairly slow.
Speaking of Mac OS X, it has some problems as well. Running Mac OS X on any virtualization or emulation platform is difficult regardless, but due to the measurements Apple has taken to fight piracy, and lock it's operating system to just Mac owners, this makes it extremely difficult to run properly on QEMU. However, I would think QEMU and VirtualBox should both be able to run the operating system by now, since Mac OS has been standardized in terms of hardware and simplified in terms of software. Emulating hardware is difficult though, so I don't necessarily blame the developers so much as I blame the lack of effort on Apple to provide support for emulation and virtualization to their platform so that developers and testers can run tests quickly and make sure Apple support is finalized before the next release.
In addition, the GUI's that are available for QEMU are either outdated, don't allow use of all of QEMU's features, or are extremely difficult to look at or understand. This wouldn't be a problem with the original team normally, but I think QEMU really deserves a GUI. VirtualBox, VMware, Bochs, and PCem all have great GUI's. So I think a GUI in Qt or WxWidgets or something would really benefit this application.
Recommendations to others considering the product
I would consider whether or not your users will need to have a simple interface or not, and what operating system they will be running on the host machine. If they need a GUI that runs on Windows, you should probably go with something like VirtualBox, VMware, or something similar.
What business problems are you solving with the product? What benefits have you realized?
Quick testing of applications on multiple systems, web browser tests, quick operating system support tests, embedded system and open system testing, compiling for game consoles and Linux systems, support for Solaris and BSD, and so much more.
I know of more than a few business' that also use QEMU for hardware development, BIOS and firmware design, and electronic architectural projects.