For me the best Soul Drinkers novels were the second one, and the fifth. book 3 had some interesting non-Astartes characters, but the astartes characters, nevermind the whole Soul drinker subplot, essentially ruined it for me.
Book one would have worked well as satire, had it been written that way. It has a very quirky, almost Space Marine like feel to it, and it could have worked well as that book. However, with Counter's subseuqent writings, it quite obviously was meant to be taken seriously, which makes it pretty absurd. I mean you have a Renegade Chapter who went to war with the Adminstratum, the AdMech, the Inquisition and its own Chapter master Just because Sarpedon got outraged at being "dishonored." that the AdMech stole one of his toys, and his only solution is to fight back against the Imperium. That has to be the most ABSURD rationale ever - it in fact echoes the pretty much what happened with Huron Blackheart and the Astral Claws, and look how that turned out.
and then, after having fought all of the Imperium, killed their own brethern, and so on... Sarpedon decides that his chapter is blessed by the emperor because they are infected horribly with mutations, can't figure out its a Tzeentchian plot despite some very ham fisted and OBVIOUS clues (names, mutations, etc. Who taught them about Chaos exactly? Oh yeah they didnt even know about Slaanesh in book 3 as I recall.) and end up shocked and outraged that they have become pawns of Chaos. To give them their fair do they do fight back, so they're at least not willing pawns of Chaos. There's the kernel of a serious story there, I could see it, but it is executed in the most hamfisted and unbelievable manner that you cannot take the book seriously. as WR and IO noted, teh first book is full of forced choices and contrivances to drive along the plot, and it really fucks it up.
Books 2 and 5 were better.. the plotting was tighter and coherent, and survival is a blievable goal in book 2. The Soul Drinkers do something noble at the end. Book 5 is perhaps the best.. Sarpedon actualyl comes across as the "noble renegade" we're meant to believe he is, he actually does what he intends to do without fucking it up.
Books 3 and 4 are pretty much pure fail and Grimdark. Sarpedon comes to a planet to personally destroy Telos (personal failure obsessing a Chapter Master - I thought Chapter Masters were supposed to be big picture guys?), end up screwing things up for the Imperium's reclamation force by forcing the crimson Fists (and their obsesseive commander) into abandoning his alies to pursue a personal vendetta. The Crimson Fists are absolute dicks and ignore anything but the Soul drinkers, and Sarpedon makes deals iwth the Dark Eldar to achieve his ends. In the end the IG forces are obliterated as is the planet. Our Heroes!
Book 4 is even worse. Sarpedon has deliberately created a schism in his ranks by his recruiting choices, and then he gets upset because the free thinking recruits he picks don't think in the way he wnated them to. This leads to a massive dose of fuckery again, and yet ANOTHER civil war, which is topped off by et another asshole Space Marine Chapter obsessed with personal honor and destroying the Soul Drinkers rather than defending the Imperium (This time the Howling Griffons.) Not nearly as grimdark as book 3, but still pretty retarded in its own way because the "plot" such as it is arises from Sarpedon's own naivete and stupidity. Book5 makes up for this some, but having this happen towards the end of your series is NOT a good thing. I actually felt at the end of Hellforged like the Imperial Fists were simply ending the series on their own initiative to spare the Imperium and the reader further suffering.
Grey Knights is a vastly different, and superior series. I'm surprised some people didn't like it. The first novel has a well paced plot, and it all fits together. It's nothing innovative story wise, but Alaric is a believable and likeable character, we get to see lots of different facets of the Imperium, there's a sense of urgency set against the backdrop of a greater catastrophe, and there are plenty of likeable and even noble characters, and even some good twists I liked. The second novel IS the weakest of the series, true, but I ascribe that to the locale it was set in and the fact it couldn't decide upon following the space battle or the ground fight. THe confrontation at the end was very consistent with the theme of the series, and Alaric's handling of it wsa very much in character. Hammer of daemons is better, even if it does create some consternation over the idea that *gasp* a GK might actually fall (AGAINST CANON!) - I rather liked counter playing with that idea because it really begs the question of what constitutes "falling" to chaos in 40K.
I also agree with WR - I despite Counter's portrayal of plasma weaponry (although he's not the only author to do it, he is among the more visible and consistent).. and his sister Aescarion story is some of his best work. (Ironic that, since that theme tends to be one he revisits and even repeats in most of his novels... Hammer of Daemons definitely had a strong feel of the Aescarion story.) Chaos tends to be one of those themes Counter is fascinated with, and I suspect its one reaosn why he gets so hit and miss. it's like Goto and the Eldar.