Well, allow me to address your comments in reverse order. First off, I wouldn't recommend using ReactOS as your main OS right now simply because it is alpha like you said. It is not intended for everyday use yet. The case in point being that on their website they said that right now their support for TCP/IP is still broken if not incredibly slow, which will put a huge damper on everyday use.
Also, I may be wrong, but I don't think the kernel would be written in java since that would require the use of a JVM to run the OS and there is no point in having all that extra overhead for an actual operating system. Traditionally, an OS will be written in C if not in outright Assembly language.
Lastly, just as it says on their website, React OS is in NO WAY a Unix-like system (it has no linux or unix code in it and NO windows code). The kernel is written completely from scratch with the sole purpose of creating an open source windows clone. The best way to put this into perspective for you is that ReactOS is essentially what GNU (and all GNU variants) is to Unix: an open source clone of the OS that aims at total binary compatibility and tries to enhance and outperform the OS that 'inspired' it. ReactOS tries to implement the API of windows in order to try to be compatible, while also trying to be better and faster. In essence, ReactOS is trying to be a better windows than anything put out by MS.
All of this is listed in the FAQ over at the reactOS webpage, so if you'd like to fact check my statements, feel free.
ReactOS is an interesting project, and I fully intend to use it as a windows replacement to run my windows programs, but until then i'll stick to dual booting linux/windows.
-Zorak