I think the first part of Balboa, between highway 5/mission blvd (at the bottom) and Clairemont drive is pretty horrible, mostly due to a combination of factors. The length and gradient, which aren't too bad, are only part of the equation:

Wide open, exposed
Fast Traffic w narrow-ish lanes and bike lane
Boring, graded road w little to no changes in direction or gradient
Trash,
Vagrants walking up and down,
makes for a pretty miserable few minutes.

I would much, much rather climb something like Torrey Pines than Balboa Ave, although that doesn't say a lot. I'd rather climb most roads, even much steeper ones, then Balboa.

Unfortunately, I ride up Balboa all the time, as my apt is in the Mesa College/Clairemont area and I ride to Fiesta Island or up the coast a lot.