The Kit: I was pleasantly surprised when I opened up the Aegis Defence line by the fact that apart from the gun emplacement, all the sections were just singular solid pieces of plastic, so no need for assembly and very little chance of breakage. The downside is that you only get the quad gun. If you wanted to build an icarus lascannon you'll have to carv it out of plasticard or give up and throw the kit back at your LGS manager's face while howling in dispair.
Painting: Do you have a pot of Leadbelcher silver and a big fat brush? Then you're pretty much there already. To give it a little bit more variance of colour I coated it in devlan mud (or whatever it's called now) and then splotchily dabbed red, blue and green wash over different bits of it. Finally I slapped black soot weathering powder on it and the bottom edges had muddy-brown weathering powder applied to make it look like it had got shit all over it from the battlefield.
Lastly I got various bits of green vegetation looking stuff (I think the stuff I bought is really supposed to be used for scenery on model train sets or something) and glued them into the nooks and crannies to make it look like the defence line had been sat in a warzone of a long grinding campaign and so stuff had started to grow on it.
Gaming: I measured the defence line to be slightly over 28" long when you stretch it out. It seems you'll be able to get the required 25% of an infantry model obscured by it, even from a flyer's point of view as long as the model is in base contact with it. Terminator sized models may find it problematic to get cover saves reliably from it, but from a level view they will be fine.
However, it's difficult to get a rhino sized model a cover save from it and pretty much impossible from an elevated POV (i.e. a flyer's POV). The situation is even worse for dreadnoughts, though again, still possible.
Finally, it's worth mentioning the effectiveness of the 50pt quad-gun. My shaky mathhammer tells me that if fired by a BS 4 model, it should strip around 1 hullpoint from an armour 12 flyer or around 0.8 hullpoints if the flyer takes a jink save. The chances of it getting an explodes result on such a model are less than 1 in 10.