There isnt much bad that I see about it but it isnt supported by most vanilla kernels, so for example to get it on ArchLinux I spent a little time to get that into a kernel, only later to find out I forgot my virtualbox modules. Hassle.

I think the idea of unionfs should be in a kernel.

See antocv's message, that's what I meant more or less.

I don't know how good aufs is (nor do I care really), but if it's not in mainline/upstream then it'll never be taken seriously.

If you ask me to start building custom kernels to support your stuff, then I'm not going to like you. :-)