As a set of C developers, Team Black Fox very much appreciates the work of Haiku, I (Black Fox, founder) use Magnussoft Zeta currently on my legacy x86 boxes. Haiku does intrigue me, BUT there are a number of things I'd like to suggest be made priority:
Multi-User support - Magnussoft Zeta's 1.51 release supports this - I think adding that as well as a root/baron user would vastly improve security and familiarity. In Haiku's current state, this aspect reminds me of DOS or AmigaOS, two OSes that do not support multi-user.
LLVM support - as a C developer I HIGHLY prefer to use Clang/LLVM as I find it handles errors better. I tried compiling it on Haiku R4 but because the main compiler is GCC 2.95 it refused to build. I could not find a way to use GCC 4, but that may be just me being stuck in UNIX land where you can change environment variables easily.
x64 support - I think after the x64 port is usable ( currently it kernel panics ), you should change the primary focus to that, but thats just my opinion as most users have legacy x86 boxes as secondary machines. In my case, I keep hardware till it fails catastrophically or I sell it, but I am not the norm, I suppose.
Anyways, love the work thus far. I hope to contribute some software, but I'm the only developer who shows interest outside of UNIX/Linux currently.
~Black Fox