Holley's new Avenger EFI/TBI and MPFI systems are getting good reviews from what I've seen. The following gentleman seems very happy with his and gave a very extensive review along with videos:
http://www.chevelles.com/forums/showthread.php?t=326671
I believe the Holley Avenger EFI/TBI system is reviewed in the latest Chevy Hi Performance magazine also.
I've read that the Holley Avenger MPFI intake manifold(oval port) is a bit restrictive for high-flo applications that are pushing the 600-650HP limits of the system, and that a lot of port work is necessary get the intake opened up properly. Holley is supposedly aware of it and is retooling to create a better oval port manifold, but there's no telling when that will be done. You might be able to replace it with Edelbrock's MPFI manifold, but then you'd probably need their fuel rails also. The Holley EFI/TBI system uses your existing manifold so it's not an issue with that system. I'm not sure about the rectangular port manifold.