Just thought I'd throw out my experience...
I just bought a double stroller last week, finally.
My son is 4.5, the baby is 2, and then expecting the new one in September. The kids will all be exactly 2.5 years apart. I bought a Mountain Buggy Urban Double off craigslist for a smokin' deal, which is an all-terrain stroller that is narrow enough to fit through any door.
I wouldn't think just "any" double stroller would be worth it. The one I bought can really "do it all" for me, and it has a 120 pound weight limit so my older boy can ride too. A smaller, flimsier double with little plastic wheels would be useless for walks or the zoo or anything but the mall, basically, and mine steers so well that I actually find it easier to maneuver than a typical single.
I am a babywearer, so my kids don't go in the stroller typically until they're much older (one year or so) and I did not need a double before because my son was exceptionally mature and obedient when his sister was born and was very good to walk by me, didn't need to hold a hand or anything. My daughter is *not* like that, and since she requires containment and will not always consent to be carried
: into the stroller she goes. Of course, when she's in the stroller then my son wants to be too, so it was easier to wrangle a double stroller then listen to them argue about who gets to ride in the stroller every time we leave the house!
When the new babe is born, I figure he/she will ride in the wrap for a few months and then my son will be bumped from the stroller. I'm thinking of having some kind of ceremony when he turns 5 or something...
O=He's just a cuddly little guy - he still loves to be worn too, although that's hard when I'm pregnant.
Anyway, the short version is, it really depends on your lifestyle and your kids' personalities. I have really liked having my double stroller since I got it, although I resisted for a long, long time.