I've printed a few decals using a color laser printer (decals printed on an inkjet printer must be sealed with a varnish before use). It's not especially difficult-- generating suitable artwork is the hardest part-- but as you've pointed out, the inability to print white without investing in a special printer can be problematic. Many commercial decals are printed over a white background so the colors show up better. I've used white decal paper, but there are two potential problems that I ran into. First, one must cut the decal very carefully and precisely to avoid a white border (assuming a white border is not part of the decal's design), and second, even if one is able to perfectly cut the decal, a white edge may still show up in the thickness of the decal. If you print on clear decal paper, the decal may not show up well if it is applied to a dark model, though clear paper works great for decals applied to white or lighter-colored models.
Here are some clear-paper decals I printed (13EEE and Panzón Customs) on a Hot Wheels car I repainted:
The dark printing shows up OK on the lighter background, but the self-printed decals don't show up as well as the small commercially printed decals.
And here is a decal (Panzón Customs) printed on white decal paper on a 1/25 scale model:
I simply printed on the white paper, and then just cut out a rectangle to the size I wanted to look like a vinyl sign on the side of the car. I think it looks as good as the commercially printed decals, but of course, it's a very simple decal.
Anyway, all I can tell you from my limited experience is that the pros of printing one's own decals are that you can have decals with your own designs , and any size you choose. The cons are that for the decal to look good on the finished model, one must carefully design the decal, keeping in mind the type of decal paper being used, the color of the paint used on the model, and the intricacy of the decal's outline (especially for small decals) if using white paper.
