The Petherbridge Wimseys follow the books less closely than the Carmichael Wimseys; there are still huge chunks of verbatim dialogue, but these later ones move scenes around a bit more freely, not really to the detriment of the story as seen on screen.
As far as casting goes, my main "hey!" moment in Strong Poison was the casting of Miss Climpson. She wasn't elderly at all. A perfectly fine and suitable actress played her, but she didn't even have grey hair. And I think the Freddy Arbuthnot actor was too young; I always envisioned him as a contemporary of Peter's (I didn't go back in the books to see if I was correct or not). But Harriet Walter makes a wonderful Harriet, even without a deep, husky voice and with bad makeup, neither of which is her fault. Ditto Edward Petherbridge, who is too old for the role but otherwise perfection in one of Wimsey's tailored suits. I could take or leave the actor who played Parker; he was good, but I didn't feel the warmth between Parker and Wimsey that I would've liked. Bunter, however, I adore. I can easily seeing Bunter being as young and cute as Richard Morant; otherwise, how did he charm all those housemaids throughout the books? The character must be close to 40 by Strong Poison (in Busman's Honeymoon, Peter mentions Bunter's "20 years service") but that didn't bother me as much as Peter looking ten years older than his supposed age. Perhaps because it's easy to imagine nothing ever changing Bunter.
J. and I disagreed a bit on Petherbridge--she feels he doesn't do the "silly ass" act well enough, but I think that act is much less present in the Harriet books anyway. And with the more serious scenes, he's simply outstanding. (Take, for instance, the Big Fight scene in Have His Carcase--wow!!!)
I think the adaptation of Strong Poison is worth watching. It's a very static book--lots of courtroom scenes and sitting around--so some cutting for television was necessary, and really, they cut very little; the main thing I missed was Peter going to Charles Parker to ask him why he hadn't yet proposed to Mary. Freddy Arbuthnot's engagement was likewise cut. The bohemian party is considerably smaller and less raucous; they made up for this by having one of the women characters be extremely butch (as she is not in the novel, unless it's so subtle I never caught it; she might be gay, but I don't think she wore men's suits). Scenes are slightly rearranged so that we are reminded throughout of Harriet in prison. There's a slight but effective change in the ending: instead of Harriet looking for Peter to thank him and finding he's driven off, he waits for her in the corridor outside the courtroom; she sees him, but turns and walks away without speaking. Same result, more dramatic visual.
Bill the lockpick made it in, and Miss Murchison, who blossoms before our eyes, and Miss Climpson's seance. The guy who played Boyes in flashback had a really sexy voice.
All of this is redeemed by the dialogues between Peter and Harriet in the prison. The camera work is very clever; at a couple of really intense moments, we see Harriet's face as she reacts to something Peter has said, but only the back of his head, so his expression is hidden. Carefully chosen moments, I would bet money. I could feel the energy between them, in this and in Have His Carcase; that's why the adaptation of Gaudy Night drives me bugfuck.