These have been much maligned in the new codex, especially with the nerf to Psycannons (i.e. Salvo rules), the loss of Psybolts, and the reduction in price of GK Terminators (making Strike Squads less valuable on a relative scale).
That said, I think Strike Squads are still a very good Troops choice, perhaps one of the best in the game, especially in comparison to their points value. Let’s compare a Strike Squad to the 40K “baseline” Troops option, the SM Tactical Squad.
Assuming you are using a CAD in both cases (so both Strike Squad GKs and Tactical Marines have Objective Secured rule), then the Strike Squad GKs and the normal Tactical Marines share the following:
-Standard MEQ stat line (WS4, BS4, S4, T4, etc.)
-Ld 8 (increased to Ld9 with Vet Sgt/Justicar)
-Power Armor/3+ save
-Access to Rhino/Razorback DTs
Now, what does the Tactical Marine have that the Strike Squad GK does not?
-Cost -6 points/model
-Access to SM special weapon (i.e. melta, plasma, etc.)
-Access to SM heavy weapons (i.e. ML, HB, Lascannon, etc.)
-Access to Drop Pods
Against this, what does a Strike Squad GK have the a Tactical Marine does not?
-Storm Bolter (better than either Bolter or Bolt Pistol, since it can shoot twice at 24″ and always shoot and assault)
-Nemesis Force Sword (with access to upgrade to Halberds, Falchions, Warding Stave, or Hammer for EVERY model)
-Psykout Grenades (moderately useful against Psykers)
-Brotherhood of Psykers (with Hammerhand and Force powers)
-Access to GK special weapons (i.e. Incinerator, Psycannons, etc.)
-The Aegis (moderately useful for improving Deny the Witch, especially when combined with Brotherhood of Psykers and Adamantium Will from a Warding Stave)
GK Strike Squads also have some very situational rules that really only help against Chaos Daemons, such as PE: Daemons, Daemonbane weapons, Banishment psychic power, etc.
Now, taking a look holistically, are Strike Squad GKs worth that extra +6 points per model?
In terms of survivability/durability, the only advantages they have over Tactical Marines are better stand-off (from Storm Bolter range) and better Deny the Witch Options (i.e. Aegis, Brotherhood of Psykers, Warding Stave, etc.).
In terms of mobility, they don’t have access to Drop Pods (unless you use allies), but they do have “organic” Deep Strike, so they can drop in from reserve for free compared to Strike Squads (but with risk of scatter/mishap and not on turn 1, unless you are using Nemesis detachment).
In terms of shooting, I already mentioned the advantage of Storm Bolters, which is significant. Even more than this is the ability to take GK special weapons. For only the price of a MB, you can take a S6 heavy flamer, which is INCREDIBLE…. even better, you can take 2 in a full 10 man squad, which as mentioned, can deep strike…. Salamander SMs would give their right arm for this ability in a Tactical squad! Psilencers are also quit a solid option if you can put your Strike Squad in cover or shooting out of a Rhino Bunker, with 12 S4 shots (that’s the equivalent of 6 rapid firing Tactical Marines) that have the ability to gain ID (even a Riptide could potentially fail a save against this with some lucky rolls and be instant-gibbed). Finally, the Psycannons… while nerfed heavily with Salvo rules, they are still extremely good value…. for three MBs each, you can take two S7 assault cannons (that can even be fired on the move, albeit at reduced capacity) in your 10 man Strike Squad…. Tactical Squads (even Devastator squads) everywhere are green with envy for this ability.
Finally, close combat… with access to Hammerhand, a Strike Squad can have 21 S6 AP3 attacks on the charge….against anything short of Terminators, that is usually enough to wipe out 8-9 enemy MEQs, meaning they can kill an entire squad on the charge in most scenarios… against hordes like Orks, they can fire 20 SB shots and then charge with another 21 S6 AP3 attacks, resulting in an average of 15 dead Orks before they even get to strike back. Against MCs, they can activate their force weapons and, if they get a single wound through (pretty easy to do with S6 AP3 Swords, S7 AP3 Halberds, and S10 AP2 Hammers), that MC goes down to ID. Against medium-light vehicles, massed S6 attacks on the charge is even better than the standard Krak grenades, while Warding Staves (S8 with Hammerhand) and Daemon Hammers (S10 with Hammerhand) will tear down even the heaviest tanks.
Now, all this is great, but is it enough to justify taking Strike Squads instead of GK Terminators? Terminators cost +13 points more per model and have all the same rules, but also have a 2+/5++ (so are significantly more survivable), have 2A base, and have relentless special weapons (so can move and fire and maximum capacity). Besides their price cost, GK Terminator disadvantages are they are bulky (for Transport purposes), cannot sweeping advance, and pay slightly more for their special weapons (though it is more than justified, based on the relentless capability TDA gives).
A kitted out squad of 10 GK Terminators with 2 Psycannons and 3 Daemon Hammers and 5 Halberds is two MBs over 400 points …. a fully kitted out 10 man Strike Squad with 2 Incinerators, 3 Daemonhammers, and 5 Falchion Pairs is a MB less than 275 points… the saving of about 140 points means that you can afford another Dreadknight in your army, or another 5 man Purifier squad, or even a second Librarian with PML(3).
Well, instead of comparing a 10 man GK Terminator Squad with a 10 man GK Strike Squad straight up, lets compare Terminator Squad with a Strike Squad and an attached Librarian with PML(3) and all Divination powers:
-The Strike Squad/Libby has 12 wounds vs the 10 wounds from the Termies, but Termies have better armor… if the Libby gets Forewarning, however, he and the Strike Squad can have a superior 4++ Invul save over the Termies.
-Strike Squad/Libby have Ld 10 and are likely to deny the witch on a 3+ (with re-rollable 1 from the Aegis), since they have PML(3) and Adamantium Will), compared to Termies, which will deny on a 5+ (with the same re-rolls of 1s from the Aegis)
-Libby and Strike Squad have 24 attacks on the charge (vs 30 for the Termies), but with Prescience from the Libby, they will make about 18 hits, which is more than number of hits that the Termies would make.
-If all models had storm bolters, Libby/Strike Squad would put out 22 shots and have an average of 19-20 hits with Prescience, compared to 20 shots and 13-14 hits for the Termies
-If the Libby had Misfortune, then he could make all the Strike Squad Storm Bolters have Rending on a specific target, making them able to even glance vehicles to death and threaten MCs/ICs far better than the Termies
As you can see, the price difference between Strike Squads and Terminators means that you can afford a Librarian at the same points cost… putting the Libby with the Strike Squad really makes it a much more potent unit and brings it up to a competitive level with equivalent-costed Terminators… same argument could be made for matching equivalent Strikes and a Dreadknight with a squad of Terminators, etc., etc.
With an elite army like GKs that puts so few models on the table, being able to add some more targets for the enemy is really a big deal and, especially in bigger games (like at the 1850 or 2000 point level), can be the difference between being wiped out turn 3-4 and being able to diffuse enemy shooting enough to survive the whole game and also destroy his units/seize objectives as required.