I have no love for these daemon machines. It used to be that the only walkers Chaos Space Marines had access to was dreadnaughts and Defilers. Dreadnaughts are another topic; this one is about the big walkers.
With the 6th edition Chaos Space Marines codex Maulerfiends and Forgefiends were added. Soul Grinders were made available as a regular unit, as opposed to an ally, in the Khorne Daemonkin book.
The fiends were added to the game around the same time Nemesis dreadknights, Tau Riptides, Eldar Wraithknights, and Space Marine centurions were added tot he game. I believe all of these models were added in response to the warmahordes game adding their collossal type models. Comparing the fiends to their opposition it is quite easy to see that they got the short end of the stick when it comes to creative rule making. All of the other large monstrous creature/walker types outclass the fiend in almost every way. The ones that can't move fast have serious defensive capabilities.
I love the defiler model, a huge crab demon that is going to blow you apart and chop up the bits. The fiends and soul grinders look good as well but I don't own any of them yet. Once again, though, these chaos models can't hold a candle to the sleek lines and sheer awesomeness that the looks opposition models have.
The defiler is too expensive, too slow, gets killed too easy, and is trying to do too much stuff at once. For a few more points other factions get better, faster, and stronger models. In eldar's case, for 95 more points they get a toughness 8 gargantuan creature that shoots 2 strength D shots a turn.
The shooty fiend is trash. Too expensive and, depending on options, can overheat and kill itself while also suffering some other negatives I will discuss further down.
The soul gridner is probably the star of the show. For the points it has better front and side armor and, depending on options, can actually shoot decent things, one of them being skyfire. The model looks great also. Once again, however, it is just too slow. Also suffering from a ballistic skill of 3 makes this thing pretty underwhelming. The only thing going for it is the armor 13 and cost compared to these other walkers. When it finally gets to combat it can probably do something, but until then the performance is substandard.
The close combat maulerfiend is the star of the pack. It moves fast, can reduce attacks or blow vehicles up easier, is fairly cheap, and looks good doing it.
The downsides of all of these models make them mediocre. All have ballistic skill/weapon skill 3. Hitting on 4s everywhere with few attacks makes these things pretty anemic. Armor 12 on most of them, with only 3 hull points, makes them one trick ponies. That lucky lascannon shot or the squad with grenades getting an immobilized result or just outright blowing it up really sucks. One lucky shot and the models is toast; 130 plus points to the dead pile or easily avoided. What really makes this hurt is that the equivalent enemy beasties simply take a wound from one of these lucky shots. Take a squad of flesh hounds instead. They will run over the marines, only lose one model to the lascannon shot, and still get an invulnerable save.
Maulerfiends are the only one of these I would consider taking, albeit reluctantly. One thing Khorne Daemonkin have trouble with is vehicles with high rear armor. The maulerfiend is the best response to it. Being that these things are fragile and cracking these tanks in close combat is almost mandatory I would rather take a few melta bombs spread out on sergeants or on a lord.
If you feel a maulerfiend is necessary, take two of them. Two is one, one is none. To the bin with everything else.