i watched a guy use D-cannon's once, 3 of them +2 wraithlords and he was beating down a tyranid carny heavy list. He shot down his zoanthropes with the cannons and also badly hurt a carnifex (which died after from other shooting). his list was very mobile, but had the D-cannons+2 gaurdian units. then as I recal he had wraithguard, the lords, farseer and maybe another unit, although it was in a small gave and a relatively small board, not to mention it was 4th edition. but they can be good.
they are a bit too defensive for my liking, but if you had them on a home objective, they could blow open rhinos, maybe kill dreadnoughts that deep struck, instakill crisis suits, but also they could get shot down. but if they were fortuned they could survive along with the help of some good target saturation.
against mobile armies d-cannons could be great, especially for killing nobz in mega armor or on bikes. if you really want to use them, you will just have to make a list that is focused on them.
you will have to have units moving forwards to not let the artillary target the cannons, and then have some units around it to not make it a delectable deep-strike+crush unit. any units that enter mid-range of your objective can then be eliminated.