I like stormboyz in their own separate squad. I usually take a couple of formations. Things I like about them are:
1) Cheap formation, so good for boosting your number of activations, giving you the chance to control the game.
2) Infantry formation, so can take full advantage of cover and can garrison.
3) Fast and can jump over stuff, so very flexible for redeploying and useful for last turn grabbing/contesting of objectives.
4) Scout special rule means that they are even more flexible. They can string out in a long chain to hold two objectives at once (not very robustly, but that might not matter on the last turn). They can string out in a long chain to get right in the face of an important enemy formation or two, stopping them moving towards objectives or forcing them to clear the scouts away and waste their turn. They can screen valuable units. They can use the annoying 10cm scout ZoC to force an enemy to move when it might not want to (eg artillery that wants to fire indirectly).
5) They can be great to trigger assaults, bringing in the firepower of something more powerful. They won’t beat up much on their own, but the following can work nicely. Double an uge kult of speed full of skorchas to within 15cm of an enemy and shoot at them. The shooting may not kill much, but it will place blast markers. Then retain with the stormboyz (on a 2+) and engage the softened up enemy (you don’t even need to get in to base to base, even though the stormboyz are poor at firefights). Just be careful to set it up so the stormboyz don’t all get killed and then you will be able to call on the awesome firefight support of the kult of speed. Stormboyz are good at this because they are manoeuvrable enough to give you a pretty good range for triggering assaults, even with a single move. And it’s certainly satisfying to get to use the kult of speed’s firefight after it’s already moved 70cm and shot in the turn
.