Yeah, Gaunt's Ghosts start out good, but I feel Abnett got a bit lazy and was pumping them out after a while.
I'd second Matt Farrer's Enforcer series. Gav Thorpe is a bit hit and miss, but his Dark Angels books were mostly hit, with the exception of the end of the last one, and all of the one about Azrael.
Ben Counter's Grey Knights and Soul Drinkers series also started out well, but went downhill a bit. Bill King's Space Wolf series also started out well (though the first and especially the second dragged at times), but then he left and they gave it to people with no writing experience but were big fans and it was rubbish.