I have one (installed in 2003 when I re-did this house). I would not have one again. The problems aren't major but they can be annoying:
1. Because the window extends out past the house's eaves and gutters, it can get quite battered by rain, and snow can sit on the top of the garden window. I have had problems with leaks in the top outer corners.
2. Although the window comes with shelves, I never used them because if you have things on the shelves... you can't see out the window very well! Because I do have a view of the garden and don't want to lose that, I don't use the shelves.
3. If your sink/counter bumps out (toward you), you won't be able to reach the outer part of the garden window without either climbing on the kitchen counter next to the window, or standing on a step-stool so that you can reach out into the window. If your countertop doesn't bump out at the sink, it may not be as bad but you'll still need to climb on the countertops to reach the fixed glass to clean it, and possibly to reach plants closest to the glass for watering too, unless you use a very long-handled watering can.
4. The 'sill' of the window (where your plants sit) gets very cold in winter. I had the kitchen countertop people make a separate piece of Corian to fit into the window so that the wood won't get wet or need scrubbing, but it still gets cold in winter. When I start seedling trays and grow them on, I have to put a seedling heat-mat in the window underneath the trays to keep them warm enough.
5. If your garden window doesn't have side casements (is just a fixed window on all three sides) your plants will broil in a west or south exposure.
5a. If the window DOES have side casements, plants can be knocked over by the wind coming through them UNLESS the side windows are hinged on the outside (away from the house). Side windows with hinges on the 'house' side can funnel too much wind into the garden window.
p.s. - I just realized you are in CA, so the snow/cold factors won't apply to you (lucky girl!) but I left them in just in case anyone from a colder climate is interested in the answers.