I've dealt with Dynamic and SystemQ and both have been pretty good to deal with. Dynamic do some very solid domes and some nice budget dvrs and SystemQ do the DVR365 which I have on test at the moment.
From talking to others LJD/beltronics aren't great and I have heard mixed reviews about COP.
EZcctv have very competitive prices for the Geovision cards etc. I was impressed by them at the IFSEC but have not ordered from them yet