I really like the water transfer decals used for models also found at hobby shops.
Cons:
1. They are fragile. It does take a little care to apply these. Some have said too much, but I think they are actually easier than dry transfers.
2. Expense? From $5 - $8 per sheet, these can be a little pricey, especially if you
have to have specific logos. I bought a few sheets of decals intended for much larger scale models to have some specific decals that I wanted.
3. They can come off if not sealed with a top coat of gloss. There is a thin layer of glue that is intended to keep these on, but when positioning them, it is easy to wash all the 'glue' away.
Pros:
1. Availablilty. Just about every model / hobby store will have sheets of automotive decals (intended for larger NASCAR models and RC cars). And they are going to have most of the name brand manufacturers for autombiles / automotive products that you would see in real life.
2. Flexibility. Because you slide these on wet, you can move them around a bit if you don't like how they are positioned. You can even remove them relatively easily even after they have dried by rewetting (assuming you haven't sealed them in gloss coat.)
3. Looks. I think these look better than anything else.
Brand logos are dead-on accurate and the decal itself is very very thin. No heavy, paper-based sticker is going to look this good. Dry transfers are a close second IMHO, but as Latency noted, he could only find them for railroad sets, and had to improvise a bit.