I completely agree that the time of year plays a huge role in effectiveness.  My best results always come in the late fall.  I will even go a step further than MiloFarmer did:  I like to wait until we have had a light freeze or two (if possible) before spraying.  Bindweed lays on the ground, and the ground's temp. keeps in from dying (for a while, of course).  Bottom line: the later the better.

During the spring/summer months, about all you're going to get is a burn-down....in my opinion, save your money because you are not going to get much long-term benefit/control.  Some LV6 (circa ~0.5 lb) with some Ally (if you can) or a shot of Banvel will get 'er done.