Upgradability is likely to be limited. Motherboard is custom and is BIOS locked so there can be chances of some gpu not being compatible. Power supply is probably weak and just enough for the pc in its current state.
Pc is good for casual uses such as browsing, work or casual uses.
Case airflow is not great but ok.
If you plan to add a gpu later on, best to build one yourself than getting a prebuilt.
What I don't understand about the "build my own" is that the processor i5-4440 and windows 8 alone are almost $300, add a case. mobo, hard drive, PSU, optical drive and it would cost much more than $499, no?
Yes it will cost more at first, but if you consider the things you need to upgrade to the prebuilt that is more expensive if you are planning to add a GPU in the future.
Building your own allows you to handpick your parts that offers better upgradability.