Not at all, really. The biggest part of the thing is to key it to the right terrain. Picking that bush stuffed way off in a corner? Yeah, dumb plan. Picking the ruined building just out ahead of his deployment zone, the one his whole force has to pass by? Real potential to cause tons of damage. Thinking ahead about where he's likely to deploy any particularly juicy targets (say, a guard player with a few Basilisks) can also help you make the strike count.
Handy (if slightly cruel) trick: Roads/Rivers/etc.. do count as a single piece of terrain, so you can plot the strike to the road down the middle of the table and drop your shots at any location along the whole length of the road! :tongue:
Then there's the matter of the scatter. While it makes it less than reliable to hit specific targets, that same scatter means that anything within 12 inches of that terrain in any direction could potentially have a big heap of Ordinance Barrage pain coming their way at any time. That's more than 3 square feet of tabletop under constant threat.
Finally, you are not required to tell your opponent what terrain you've picked. Write it down, and keep it secret until you start dropping templates. Even just the knowledge that he could be facing S10 AP1 pieplates o' doom dropping near any terrain you choose can put any opponent seriously on edge.
How has it worked for me? Hit and miss, really. Some games it just pulverizes empty fields. Usually it will get some partial hits on a few units, maybe a weak hit on a tank, just enough to cause some chaos. Occasionally, it really rocks. I've seen it Vaporize a Vindicator, Destroy a Dreadnought, and Terminate a full Termie squad all in one game before. (Sorry for the corny alliteration, everybody... <_< ) In short, if you pick a good terrain piece, try to keep his troops stuck in the kill zone if possible, and get some lucky scatter rolls, they can be devastating. (or they can be useless. Nature of the beast, really...)
I've used Orbital Strike, I recommend it to psyche out your opponent but besides that...it's useless. Besides the fact that you will (most-likely) forget about it, its special rule is (in)accuracy so it will barely ever hit...
As has been said above, it's a great psychological weapon. Don't buy an OS expecting it to make its points back (or even kill anything at all). Buy an OS with the intention of throwing a wrench into the enemy's plans. Not knowing what terrain piece the Strike is aimed at or when it comes in definitely makes the enemy play cautiously. On tables with roads or rivers, an OS can be brutal - you may retarget the exact location of the OS every turn, as long as it's based on the same terrain piece. With a long piece like a road, you can effectively threaten the entire table with one Strike!
As has been stated by earlier posters, an OS is more a psychological weapon to deny an area to an enemy. However, with a little bit of luck it can be extremely useful and deadly. I once placed one on a hill in the middle of the table slightly in my opponent's half. He was playing space wolves. I sat back in the back half of my zone, for the most part, and shot at him while he advanced. He advanced straight over the hill just as my OS landed. The entire hill was covered with loaded rhinos and/or ground troops. I had to be UNLUCKY to miss. He couldn't scatter fast enough to escape the scatter of the OS and I wiped out almost half his troops just from the OS.
Now, I have also had games where my OS didn't kill a thing. However, it still worked well to convince my opponent to NOT set up his devestators on a certian hill, or NOT advance down a certain direction, etc.
One tactic that I have been considering trying is a GKT-heavy force then use a Melta Torpedo. With AP3, it will allow GKT saves while denying the majority of other models their saves (as well as being dangerous to vehicles). DS the GKT as well as place the OS on the same spot. Sure, you might lose a GKT or two, but it will just obliterate your opponent. I think it will work especially well on a scenario with an objective.
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