Strategic bombers work best against cities only if there is no danger of your airfields being over run, like on an island or accross a channel. Ciites repair themselves and are normally heavilly defended, so this is a high maintenance approach. Strategic bombers are better suited for destroying enemy factories which must be rebuilt for 80 pps or for getting that all important bridge in the enemy rear and frigging up his supply and arms distribution. This has been my own experience anyway.
To be honest, most of the time I feel that an air force is a VERY expensive luxury. When one of my opponents suddenly has a large airforce, I am sure to suddenly have a lot more tanks, and I know which one I would rather have. :-) Airforces burn oil like its free. In most of the games I play, everyone is running flat out and there is not enough oil for tanks, so bombers would be grounded anyway.
I have developed a very cost effective integrated fighter/flak Air Defense system that can maintain air parity (that is denying the enemy air superiority) without burning oil, which largely neutralizes the enemy investment into air power.
In summary, they are very task specific. You may only need them once every five to ten games, but they are great for killing factories and bridges.
< Message edited by Meanfcker -- 10/3/2012 1:49:43 AM >